proc: fix the issue of proc_mem_open returning NULL

proc: fix the issue of proc_mem_open returning NULL

proc_mem_open() can return an errno, NULL, or mm_struct*.  If it fails to
acquire mm, it returns NULL, but the caller does not check for the case
when the return value is NULL.

The following conditions lead to failure in acquiring mm:

  - The task is a kernel thread (PF_KTHREAD)
  - The task is exiting (PF_EXITING)

Changes:

  - Add documentation comments for the return value of proc_mem_open().
  - Add checks in the caller to return -ESRCH when proc_mem_open()
    returns NULL.

Link: https://lkml.kernel.org/r/20250404063357.78891-1-superman.xpt@gmail.com
Reported-by: syzbot+f9238a0a31f9b5603fef@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000f52642060d4e3750@google.com
Signed-off-by: Penglei Jiang <superman.xpt@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Adrian Ratiu <adrian.ratiu@collabora.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Felix Moessbauer <felix.moessbauer@siemens.com>
Cc: Jeff layton <jlayton@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>