btrfs: rename and export read_node_slot

Preparatory work for code that will be moved out of ctree and uses this
function.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 07c0883..dc465df 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2587,6 +2587,9 @@ int btrfs_find_next_key(struct btrfs_root *root, struct btrfs_path *path,
 int btrfs_search_forward(struct btrfs_root *root, struct btrfs_key *min_key,
 			 struct btrfs_path *path,
 			 u64 min_trans);
+struct extent_buffer *btrfs_read_node_slot(struct extent_buffer *parent,
+					   int slot);
+
 enum btrfs_compare_tree_result {
 	BTRFS_COMPARE_TREE_NEW,
 	BTRFS_COMPARE_TREE_DELETED,