volume_id: abort early on read failures.
 should help with probing missing fdd's

diff --git a/util-linux/volume_id/sysv.c b/util-linux/volume_id/sysv.c
index 7671962..1650332 100644
--- a/util-linux/volume_id/sysv.c
+++ b/util-linux/volume_id/sysv.c
@@ -83,8 +83,9 @@
 #define XENIX_MAGIC				0x2b5544
 #define SYSV_MAX_BLOCKSIZE			0x800
 
-int volume_id_probe_sysv(struct volume_id *id, uint64_t off)
+int volume_id_probe_sysv(struct volume_id *id /*,uint64_t off*/)
 {
+#define off ((uint64_t)0)
 	struct sysv_super *vs;
 	struct xenix_super *xs;
 	unsigned boff;