Add a test for concurrent removal of all tiered devices at once

[ojab]

Quick & dirty test, echo & fs us should be removed, join_by and alike used, dd size & cat count tweaked before merge and I don't even know if it works fine after mentioned issues are fixed.

1. Invalid option metadata_replicas: too big (max 4)
2. After making metadata_replicas=3 we'll get Invalid option data_replicas: too big (max 4)
3. After making data_replicas=3 we'll get either hang (ssh got disconnected, output stops) or (less often) backtrace, for example

bcachefs (ad5a54fa-d6bc-4b4e-9435-7e90827cf772): going read-write
DONE
------------[ cut here ]------------
------------[ cut here ]------------
btree trans held srcu lock (delaying memory reclaim) for 39 seconds
btree trans held srcu lock (delaying memory reclaim) for 20 seconds
WARNING: CPU: 1 PID: 411 at fs/bcachefs/btree_iter.c:2838 bch2_trans_put+0x4f5/0x550
WARNING: CPU: 0 PID: 53 at fs/bcachefs/btree_iter.c:2838 bch2_trans_srcu_unlock+0x150/0x160
Modules linked in:
Modules linked in:
CPU: 0 PID: 53 Comm: kworker/u17:0 Not tainted 6.6.0-ktest-03573-g6c5850f4860d #3
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
Workqueue: writeback wb_workfn (flush-bcachefs-2)

RIP: 0010:bch2_trans_srcu_unlock+0x150/0x160
CPU: 1 PID: 411 Comm: bch-reclaim/ad5 Not tainted 6.6.0-ktest-03573-g6c5850f4860d #3
Code: 46 9e cd 00 48 c7 c7 f8 28 00 82 48 b8 cf f7 53 e3 a5 9b c4 20 48 29 ca 48 d1 ea 48 f7 e2 48 89 d6 48 c1 ee 04 e8 70 f2 b3 ff <0f> 0b e9 7c ff ff ff 0f 0b 0f 0b 0f 0b eb 87 90 66 0f 1f 00 0f 1f
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
RSP: 0018:ffff88810082b9b8 EFLAGS: 00010296
RIP: 0010:bch2_trans_put+0x4f5/0x550

Code: d1 8c cd 00 48 c7 c7 f8 28 00 82 48 b8 cf f7 53 e3 a5 9b c4 20 48 29 ca 48 d1 ea 48 f7 e2 48 89 d6 48 c1 ee 04 e8 fb e0 b3 ff <0f> 0b 8b b3 90 00 00 00 49 8d bc 24 70 35 00 00 83 fe 01 77 3b e8
RAX: 0000000000000043 RBX: ffff888108ba4000 RCX: 0000000000000027
RDX: ffff88817981c3c8 RSI: 0000000000000001 RDI: ffff88817981c3c0
RBP: ffff88810082b9d0 R08: ffffffff81e8bd80 R09: 000000000002cdb0
R10: 0000000000000001 R11: ffff88810007c758 R12: ffff88810af40000
RSP: 0018:ffff888108117d38 EFLAGS: 00010292
R13: ffff888108ba6530 R14: 0000000000000003 R15: 00000000fffff790

FS:  0000000000000000(0000) GS:ffff888179800000(0000) knlGS:0000000000000000
RAX: 0000000000000043 RBX: ffff888105544000 RCX: 0000000000000027
RDX: ffff88817985c3c8 RSI: 0000000000000001 RDI: ffff88817985c3c0
RBP: ffff888108117d68 R08: 0000000000000003 R09: ffff88817dbfe000
R10: 0000000000000001 R11: ffff88817dbfe000 R12: ffff88810af40000
R13: ffff88810af40000 R14: ffff88810af434a0 R15: ffff88811692e170
FS:  0000000000000000(0000) GS:ffff888179840000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f60f6e6ee84 CR3: 000000000222a000 CR4: 00000000003506a0
Call Trace:
 <TASK>
 ? show_regs+0x65/0x70
 ? __warn+0x89/0x130
 ? bch2_trans_put+0x4f5/0x550
 ? report_bug+0x159/0x180
 ? prb_read_valid+0x20/0x30
 ? handle_bug+0x40/0x70
 ? exc_invalid_op+0x1c/0x70
 ? asm_exc_invalid_op+0x1f/0x30
 ? bch2_trans_put+0x4f5/0x550
 ? bch2_trans_put+0x4f5/0x550
 bch2_btree_key_cache_journal_flush+0x1a0/0x240
 ? bch2_btree_key_cache_journal_flush+0x9e/0x240
 journal_flush_pins.constprop.0+0x183/0x2d0
 __bch2_journal_reclaim+0x2d0/0x460
 bch2_journal_reclaim_thread+0x80/0x160
 ? __bch2_journal_reclaim+0x460/0x460
 kthread+0xdb/0x100
 ? kthread_complete_and_exit+0x30/0x30
 ret_from_fork+0x3a/0x60
 ? kthread_complete_and_exit+0x30/0x30
 ret_from_fork_asm+0x11/0x20
 </TASK>
---[ end trace 0000000000000000 ]---
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6c5ed106f4 CR3: 000000000222a000 CR4: 00000000003506b0
Call Trace:
 <TASK>
 ? show_regs+0x65/0x70
 ? __warn+0x89/0x130
 ? bch2_trans_srcu_unlock+0x150/0x160
 ? report_bug+0x159/0x180
 ? handle_bug+0x40/0x70
 ? exc_invalid_op+0x1c/0x70
 ? asm_exc_invalid_op+0x1f/0x30
 ? bch2_trans_srcu_unlock+0x150/0x160
 ? bch2_trans_srcu_unlock+0x150/0x160
 bch2_trans_begin+0x5da/0x6a0
 bch2_write_inode+0x79/0x200
 ? bch2_getattr+0x130/0x130
 ? bch2_inode_peek_nowarn.isra.0+0x92/0x100
 bch2_vfs_write_inode+0x49/0x80
 __writeback_single_inode+0x24d/0x2d0
 writeback_sb_inodes+0x1a4/0x450
 __writeback_inodes_wb+0x54/0xf0
 ? queue_io+0xf1/0x100
 wb_writeback+0x233/0x280
 wb_workfn+0x2dc/0x410
 ? __switch_to+0x131/0x460
 process_one_work+0x138/0x2c0
 worker_thread+0x2ea/0x420
 ? rescuer_thread+0x400/0x400
 kthread+0xdb/0x100
 ? kthread_complete_and_exit+0x30/0x30
 ret_from_fork+0x3a/0x60
 ? kthread_complete_and_exit+0x30/0x30
 ret_from_fork_asm+0x11/0x20
 </TASK>
---[ end trace 0000000000000000 ]---