mds: clear setuid/setgid bits on ownership changes

mds: clear setuid/setgid bits on ownership changes

If we get a ownership change, POSIX mandates that you clear the
setuid and setgid bits unless you are "appropriately privileged", in
which case the OS is allowed to leave them intact.

Linux however always clears those bits, regardless of the process
privileges, as that makes it simpler to close some potential races.
Have ceph do the same.

Signed-off-by: Jeff Layton <jlayton@redhat.com>