Btrfs: optimize btrfs_lookup_extent_info()

If we're looking for a metadata item in the tree and the
search fails with return value of 1, and the slot doesn't
point to the first item in the leaf, check if the previous
item in the leaf corresponds to an extent item for the same
object id - if it does, then don't do another tree search
to get it.

This optimization is already done by btrfs-progs.

V2: updated commit message.
Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/extent-tree.c

@@ -771,11 +771,24 @@ int btrfs_lookup_extent_info(struct btrfs_trans_handle *trans,
 		goto out_free;
 
 	if (ret > 0 && metadata && key.type == BTRFS_METADATA_ITEM_KEY) {
+		metadata = 0;
+		if (path->slots[0]) {
+			path->slots[0]--;
+			btrfs_item_key_to_cpu(path->nodes[0], &key,
+					      path->slots[0]);
+			if (key.objectid == bytenr &&
+			    key.type == BTRFS_EXTENT_ITEM_KEY &&
+			    key.offset == root->leafsize)
+				ret = 0;
+		}
+		if (ret) {
+			key.objectid = bytenr;
 			key.type = BTRFS_EXTENT_ITEM_KEY;
 			key.offset = root->leafsize;
 			btrfs_release_path(path);
 			goto again;
 		}
+	}
 
 	if (ret == 0) {
 		leaf = path->nodes[0];