helper for mount rootwards traversal

The loops in follow_dotdot{_rcu()} are doing the same thing:
we have a mount and we want to find out how far up the chain
of mounts do we need to go.

We follow the chain of mount until we find one that is not
directly overmounting the root of another mount.  If such
a mount is found, we want the location it's mounted upon.
If we run out of chain (i.e. get to a mount that is not
mounted on anything else) or run into process' root, we
report failure.

On success, we want (in RCU case) d_seq of resulting location
sampled or (in non-RCU case) references to that location
acquired.

This commit introduces such primitive for RCU case and
switches follow_dotdot_rcu() to it; non-RCU case will be
go in the next commit.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1134,6 +1134,26 @@ int follow_up(struct path *path)
 }
 EXPORT_SYMBOL(follow_up);
 
+static bool choose_mountpoint_rcu(struct mount *m, const struct path *root,
+				  struct path *path, unsigned *seqp)
+{
+	while (mnt_has_parent(m)) {
+		struct dentry *mountpoint = m->mnt_mountpoint;
+
+		m = m->mnt_parent;
+		if (unlikely(root->dentry == mountpoint &&
+			     root->mnt == &m->mnt))
+			break;
+		if (mountpoint != m->mnt.mnt_root) {
+			path->mnt = &m->mnt;
+			path->dentry = mountpoint;
+			*seqp = read_seqcount_begin(&mountpoint->d_seq);
+			return true;
+		}
+	}
+	return false;
+}
+
 /*
  * Perform an automount
  * - return -EISDIR to tell follow_managed() to stop and return the path we
@@ -1696,23 +1716,11 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd,
 	if (path_equal(&nd->path, &nd->root))
 		goto in_root;
 	if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) {
-		struct path path = nd->path;
+		struct path path;
 		unsigned seq;
-
-		while (1) {
-			struct mount *mnt = real_mount(path.mnt);
-			struct mount *mparent = mnt->mnt_parent;
-			struct dentry *mountpoint = mnt->mnt_mountpoint;
-			seq = read_seqcount_begin(&mountpoint->d_seq);
-			if (&mparent->mnt == path.mnt)
-				goto in_root;
-			path.dentry = mountpoint;
-			path.mnt = &mparent->mnt;
-			if (path_equal(&path, &nd->root))
+		if (!choose_mountpoint_rcu(real_mount(nd->path.mnt),
+					   &nd->root, &path, &seq))
 			goto in_root;
-			if (path.dentry != path.mnt->mnt_root)
-				break;
-		}
 		if (unlikely(nd->flags & LOOKUP_NO_XDEV))
 			return ERR_PTR(-ECHILD);
 		nd->path = path;