Patch from Jakub Jelínek <jakub@redhat.com>

Make sure setfsuid/setfsgid return values are right.  Before
include/linux/security.h was added, setfsuid/setfsgid always returned
old_fsuid, no matter if the fsuid was actually changed or not.

With the default security ops it seems to do the same, because both
security_task_setuid and security_task_post_setuid return 0, but these
are hooks which seem to return 0 on success, -errno on failure, so if
some non-default security hook is installed and ever returns -errno in
setfsuid/setfsgid, -errno will be returned from the syscall instead of
the expected old_fsuid.  This makes it hard to distinguish uids
0xfffff001 ..  0xffffffff from errors of security hooks.