btrfs: expand btrfs_find_item if found_key is NULL

If the found_key is NULL, then btrfs_find_item becomes a verbose wrapper
for simple btrfs_search_slot.

After we've removed all such callers, passing a NULL key is not valid
anymore.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 6182e54..8d48660 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1631,6 +1631,7 @@
 {
 	struct btrfs_root *root;
 	struct btrfs_path *path;
+	struct btrfs_key key;
 	int ret;
 
 	if (location->objectid == BTRFS_ROOT_TREE_OBJECTID)
@@ -1675,8 +1676,11 @@
 		ret = -ENOMEM;
 		goto fail;
 	}
-	ret = btrfs_find_item(fs_info->tree_root, path, BTRFS_ORPHAN_OBJECTID,
-			location->objectid, BTRFS_ORPHAN_ITEM_KEY, NULL);
+	key.objectid = BTRFS_ORPHAN_OBJECTID;
+	key.type = BTRFS_ORPHAN_ITEM_KEY;
+	key.offset = location->objectid;
+
+	ret = btrfs_search_slot(NULL, fs_info->tree_root, &key, path, 0, 0);
 	btrfs_free_path(path);
 	if (ret < 0)
 		goto fail;