module: zfs: fix unused, remove argsused

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12844

module/zfs/vdev_mirror.c

@@ -880,6 +880,8 @@ static uint64_t
 vdev_mirror_rebuild_asize(vdev_t *vd, uint64_t start, uint64_t asize,
     uint64_t max_segment)
 {
+	(void) start;
+
 	uint64_t psize = MIN(P2ROUNDUP(max_segment, 1 << vd->vdev_ashift),
 	    SPA_MAXBLOCKSIZE);