btrfs: merge back btrfs_read_lock_root_node helpers

We no longer have recursive locking and there's no need for separate
helpers that allowed the transition to rwsem with minimal code changes.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 2a53887..8148007 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2533,7 +2533,7 @@ static struct extent_buffer *btrfs_search_slot_get_root(struct btrfs_root *root,
 		 * We don't know the level of the root node until we actually
 		 * have it read locked
 		 */
-		b = __btrfs_read_lock_root_node(root, 0);
+		b = btrfs_read_lock_root_node(root);
 		level = btrfs_header_level(b);
 		if (level > write_lock_level)
 			goto out;