Fix waitpid UAF, correct PR_SET_MM handling, add mincore stub

[sunhaosheng]

1. read child pid/exit_code before free in waitpid

In sys_waitpid, child.pid() and child.exit_code() were read after calling child.free(), which could lead to use-after-free issues.

Fix: Read pid and exit_code before calling free().

2. PR_SET_MM Handling in prctl

File: api/src/syscall/task/ctl.rs

Bug Description:
The prctl syscall was incorrectly matching PR_SET_MM_* constants at the top level instead of as sub-options of PR_SET_MM.

Fix:
Restructured the match to properly handle PR_SET_MM with its sub-options:

PR_SET_MM => match arg2 as u32 {
    PR_SET_MM_START_CODE | PR_SET_MM_END_CODE | ... => {}
    _ => return Err(AxError::InvalidInput),
}

3. Added mincore Syscall Stub

File: api/src/syscall/mm/mmap.rs

Description:
Added sys_mincore that returns ENOSYS (Unsupported). This is safer than returning incorrect data and lets programs use fallback logic.