mnt: On an unmount propagate clearing of MNT_LOCKED

A prerequisite of calling umount_tree is that the point where the tree
is mounted at is valid to unmount.

If we are propagating the effect of the unmount clear MNT_LOCKED in
every instance where the same filesystem is mounted on the same
mountpoint in the mount tree, as we know (by virtue of the fact
that umount_tree was called) that it is safe to reveal what
is at that mountpoint.

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/fs/pnode.h b/fs/pnode.h
index aa6d65d..af47d4b 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -42,6 +42,7 @@
 		struct hlist_head *);
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct mount *, int);
+void propagate_mount_unlock(struct mount *);
 void mnt_release_group_id(struct mount *);
 int get_dominating_id(struct mount *mnt, const struct path *root);
 unsigned int mnt_get_count(struct mount *mnt);