lightningd: fix occasional memleak when we detach subd from channel.

Do this by setting notleak when we do the detach!

```
**BROKEN** lightningd: MEMLEAK: 0x60f0000bbb38
**BROKEN** lightningd:   label=ccan/ccan/io/io.c:92:struct io_conn
**BROKEN** lightningd:   alloc:
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/tal/tal.c:488 (tal_alloc_)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/io/io.c:92 (io_new_conn_)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/subd.c:781 (new_subd)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/subd.c:835 (new_channel_subd_)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/channel_control.c:1715 (peer_start_channeld)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/peer_control.c:1390 (connect_activate_subd)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/peer_control.c:1516 (peer_connected_hook_final)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/plugin_hook.c:243 (hook_done)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/plugin_hook.c:343 (plugin_hook_call_next)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/plugin_hook.c:299 (plugin_hook_callback)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/plugin.c:701 (plugin_response_handle)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/plugin.c:790 (plugin_read_json)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/io/io.c:60 (next_plan)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/io/io.c:422 (do_plan)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/io/io.c:439 (io_ready)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/ccan/ccan/io/poll.c:470 (io_loop)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/io_loop_with_timers.c:22 (io_loop_with_timers)
**BROKEN** lightningd:     /home/runner/work/lightning/lightning/lightningd/lightningd.c:1492 (main)
**BROKEN** lightningd:     ../sysdeps/nptl/libc_start_call_main.h:58 (__libc_start_call_main)
**BROKEN** lightningd:     ../csu/libc-start.c:392 (__libc_start_main_impl)
**BROKEN** lightningd:   parents:
**BROKEN** lightningd:     lightningd/lightningd.c:108:struct lightningd
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

lightningd/subd.c

@@ -438,6 +438,8 @@ static bool handle_peer_error(struct subd *sd, const u8 *msg, int fds[1])
 
 	/* Don't free sd; we may be about to free channel. */
 	sd->channel = NULL;
+	/* While it's cleaning up, this is not a leak! */
+	notleak(sd);
 	sd->errcb(channel, peer_fd, desc, err_for_them, disconnect, warning);
 	return true;
 }
@@ -641,6 +643,8 @@ static void destroy_subd(struct subd *sd)
 
 		/* Clear any transient messages in billboard */
 		sd->billboardcb(channel, false, NULL);
+		/* While it's cleaning up, this is not a leak! */
+		notleak(sd);
 		sd->channel = NULL;
 
 		/* We can be freed both inside msg handling, or spontaneously. */
@@ -928,11 +932,6 @@ void subd_release_channel(struct subd *owner, const void *channel)
 		assert(owner->channel == channel);
 		owner->channel = NULL;
 		tal_free(owner);
-	} else {
-		/* Caller has reassigned channel->owner, so there's no pointer
-		 * to this subd owner while it's freeing itself.  If we
-		 * ask memleak right now, it will complain! */
-		notleak(owner);
 	}
 }