fs: take i_mutex during prepare_binprm for set[ug]id executables

fs: take i_mutex during prepare_binprm for set[ug]id executables

commit 8b01fc86b9f425899f8a3a8fc1c47d73c2c20543 upstream.

This prevents a race between chown() and execve(), where chowning a
setuid-user binary to root would momentarily make the binary setuid
root.

This patch was mostly written by Linus Torvalds.

Signed-off-by: Jann Horn <jann@thejh.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
 - Drop the task_no_new_privs() and user namespace checks
 - Open-code file_inode()
 - s/READ_ONCE/ACCESS_ONCE/
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>