mnt: Add MNT_UMOUNT flag

In some instances it is necessary to know if the the unmounting
process has begun on a mount.  Add MNT_UMOUNT to make that reliably
testable.

This fix gets used in fixing locked mounts in MNT_DETACH

Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/fs/namespace.c b/fs/namespace.c
index 54cbef1..d170814 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1331,8 +1331,10 @@
 	struct mount *p;
 
 	/* Gather the mounts to umount */
-	for (p = mnt; p; p = next_mnt(p, mnt))
+	for (p = mnt; p; p = next_mnt(p, mnt)) {
+		p->mnt.mnt_flags |= MNT_UMOUNT;
 		list_move(&p->mnt_list, &tmp_list);
+	}
 
 	/* Hide the mounts from lookup_mnt and mnt_mounts */
 	list_for_each_entry(p, &tmp_list, mnt_list) {