[PATCH] unbindable mounts

An unbindable mount does not forward or receive propagation.  Also
unbindable mount disallows bind mounts.  The semantics is as follows.

Bind semantics:
  It is invalid to bind mount an unbindable mount.

Move semantics:
  It is invalid to move an unbindable mount under shared mount.

Clone-namespace semantics:
  If a mount is unbindable in the parent namespace, the corresponding
  cloned mount in the child namespace becomes unbindable too.  Note:
  there is subtle difference, unbindable mounts cannot be bind mounted
  but can be cloned during clone-namespace.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/pnode.c

@@ -82,6 +82,8 @@ void change_mnt_propagation(struct vfsmount *mnt, int type)
 	if (type != MS_SLAVE) {
 		list_del_init(&mnt->mnt_slave);
 		mnt->mnt_master = NULL;
+		if (type == MS_UNBINDABLE)
+			mnt->mnt_flags |= MNT_UNBINDABLE;
 	}
 }