btrfs-progs: handle failed search in find_search_start better

The warning can pop up frequently on a fuzzed image, the message seems
to be enough. Add a more fitting error code too.

Signed-off-by: David Sterba <dsterba@suse.com>

extent-tree.c

@@ -284,7 +284,7 @@ out:
 	if (!cache) {
 		printk("Unable to find block group for %llu\n",
 			(unsigned long long)search_start);
-		WARN_ON(1);
+		return -ENOENT;
 	}
 	return -ENOSPC;