mount: Retest MNT_LOCKED in do_umount

It was recently pointed out that the one instance of testing MNT_LOCKED
outside of the namespace_sem is in ksys_umount.

Fix that by adding a test inside of do_umount with namespace_sem and
the mount_lock held.  As it helps to fail fails the existing test is
maintained with an additional comment pointing out that it may be racy
because the locks are not held.

Cc: stable@vger.kernel.org
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Fixes: 5ff9d8a6 ("vfs: Lock in place mounts from more privileged users")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>