gluster: io_u->error should be a positive error value

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/engines/glusterfs_async.c b/engines/glusterfs_async.c
index 8283f8b..7c2c139 100644
--- a/engines/glusterfs_async.c
+++ b/engines/glusterfs_async.c
@@ -122,7 +122,7 @@
 	else if (io_u->ddir == DDIR_SYNC)
 		r = glfs_fsync_async(g->fd, gf_async_cb, io_u);
 	else
-		r = -EINVAL;
+		r = EINVAL;
 
 	if (r) {
 		log_err("glfs queue failed.\n");