netfs: Fix retry for fs with no ->prepare_read()

fs/netfs/read_retry.c

diff --git a/fs/netfs/read_retry.c b/fs/netfs/read_retry.c
index bf6f265..2290af0 100644
--- a/fs/netfs/read_retry.c
+++ b/fs/netfs/read_retry.c
@@ -129,7 +129,8 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
 
 			/* Renegotiate max_len (rsize) */
 			stream->sreq_max_len = subreq->len;
-			if (rreq->netfs_ops->prepare_read(subreq) < 0) {
+			if (rreq->netfs_ops->prepare_read &&
+			    rreq->netfs_ops->prepare_read(subreq) < 0) {
 				trace_netfs_sreq(subreq, netfs_sreq_trace_reprep_failed);
 				__set_bit(NETFS_SREQ_FAILED, &subreq->flags);
 				goto abandon;