btrfs: use btrfs_read_node_slot in do_relocation

We're open coding btrfs_read_node_slot in do_relocation, replace this
with the proper helper.
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>

fs/btrfs/relocation.c

@@ -2191,7 +2191,6 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 			 struct btrfs_key *key,
 			 struct btrfs_path *path, int lowest)
 {
-	struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;
 	struct btrfs_backref_node *upper;
 	struct btrfs_backref_edge *edge;
 	struct btrfs_backref_edge *edges[BTRFS_MAX_LEVEL - 1];
@@ -2199,7 +2198,6 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 	struct extent_buffer *eb;
 	u32 blocksize;
 	u64 bytenr;
-	u64 generation;
 	int slot;
 	int ret;
 	int err = 0;
@@ -2209,7 +2207,6 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 	path->lowest_level = node->level + 1;
 	rc->backref_cache.path[node->level] = node;
 	list_for_each_entry(edge, &node->upper, list[LOWER]) {
-		struct btrfs_key first_key;
 		struct btrfs_ref ref = { 0 };
 
 		cond_resched();
@@ -2282,17 +2279,10 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 		}
 
 		blocksize = root->fs_info->nodesize;
-		generation = btrfs_node_ptr_generation(upper->eb, slot);
-		btrfs_node_key_to_cpu(upper->eb, &first_key, slot);
-		eb = read_tree_block(fs_info, bytenr, generation,
-				     upper->level - 1, &first_key);
+		eb = btrfs_read_node_slot(upper->eb, slot);
 		if (IS_ERR(eb)) {
 			err = PTR_ERR(eb);
 			goto next;
-		} else if (!extent_buffer_uptodate(eb)) {
-			free_extent_buffer(eb);
-			err = -EIO;
-			goto next;
 		}
 		btrfs_tree_lock(eb);