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/ctree.c b/fs/btrfs/ctree.c
index f54511d..20d1f2b 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -2618,13 +2618,14 @@
 	struct extent_buffer *eb;
 
 	ASSERT(path);
+	ASSERT(found_key);
 
 	key.type = key_type;
 	key.objectid = iobjectid;
 	key.offset = ioff;
 
 	ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
-	if ((ret < 0) || (found_key == NULL))
+	if (ret < 0)
 		return ret;
 
 	eb = path->nodes[0];