btrfs: use btrfs_read_node_slot in btrfs_realloc_node

We have this open-coded nightmare in btrfs_realloc_node that does
the same thing that the normal read path does, which is to see if we
have the eb in memory already, and if not read it, and verify the eb is
uptodate.  Delete this open coding and simply use btrfs_read_node_slot.
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/ctree.c

@@ -1570,7 +1570,6 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	struct extent_buffer *cur;
 	u64 blocknr;
-	u64 gen;
 	u64 search_start = *last_ret;
 	u64 last_block = 0;
 	u64 other;
@@ -1578,14 +1577,10 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
 	int end_slot;
 	int i;
 	int err = 0;
-	int parent_level;
-	int uptodate;
 	u32 blocksize;
 	int progress_passed = 0;
 	struct btrfs_disk_key disk_key;
 
-	parent_level = btrfs_header_level(parent);
-
 	WARN_ON(trans->transaction != fs_info->running_transaction);
 	WARN_ON(trans->transid != fs_info->generation);
 
@@ -1597,7 +1592,6 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
 		return 0;
 
 	for (i = start_slot; i <= end_slot; i++) {
-		struct btrfs_key first_key;
 		int close = 1;
 
 		btrfs_node_key(parent, &disk_key, i);
@@ -1606,8 +1600,6 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
 
 		progress_passed = 1;
 		blocknr = btrfs_node_blockptr(parent, i);
-		gen = btrfs_node_ptr_generation(parent, i);
-		btrfs_node_key_to_cpu(parent, &first_key, i);
 		if (last_block == 0)
 			last_block = blocknr;
 
@@ -1624,31 +1616,9 @@ int btrfs_realloc_node(struct btrfs_trans_handle *trans,
 			continue;
 		}
 
-		cur = find_extent_buffer(fs_info, blocknr);
-		if (cur)
-			uptodate = btrfs_buffer_uptodate(cur, gen, 0);
-		else
-			uptodate = 0;
-		if (!cur || !uptodate) {
-			if (!cur) {
-				cur = read_tree_block(fs_info, blocknr, gen,
-						      parent_level - 1,
-						      &first_key);
-				if (IS_ERR(cur)) {
-					return PTR_ERR(cur);
-				} else if (!extent_buffer_uptodate(cur)) {
-					free_extent_buffer(cur);
-					return -EIO;
-				}
-			} else if (!uptodate) {
-				err = btrfs_read_buffer(cur, gen,
-						parent_level - 1,&first_key);
-				if (err) {
-					free_extent_buffer(cur);
-					return err;
-				}
-			}
-		}
+		cur = btrfs_read_node_slot(parent, i);
+		if (IS_ERR(cur))
+			return PTR_ERR(cur);
 		if (search_start == 0)
 			search_start = last_block;