mnt: Factor umount_mnt from umount_tree

For future use factor out a function umount_mnt from umount_tree.
This function unhashes a mount and remembers where the mount
was mounted so that eventually when the code makes it to a
sleeping context the mountpoint can be dput.

Cc: stable@vger.kernel.org
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

fs/namespace.c

@@ -819,6 +819,16 @@ static void detach_mnt(struct mount *mnt, struct path *old_path)
 	unhash_mnt(mnt);
 }
 
+/*
+ * vfsmount lock must be held for write
+ */
+static void umount_mnt(struct mount *mnt)
+{
+	/* old mountpoint will be dropped when we can do that */
+	mnt->mnt_ex_mountpoint = mnt->mnt_mountpoint;
+	unhash_mnt(mnt);
+}
+
 /*
  * vfsmount lock must be held for write
  */
@@ -1371,9 +1381,7 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 		pin_insert_group(&p->mnt_umount, &p->mnt_parent->mnt, &unmounted);
 		if (mnt_has_parent(p)) {
 			mnt_add_count(p->mnt_parent, -1);
-			/* old mountpoint will be dropped when we can do that */
-			p->mnt_ex_mountpoint = p->mnt_mountpoint;
-			unhash_mnt(p);
+			umount_mnt(p);
 		}
 		change_mnt_propagation(p, MS_PRIVATE);
 	}