Grow: use raid_disks, not nr_disks

nr_disks is just wrong here - the arrays need room for all disk slots,
even if some are empty, plus spares, plus a possible backup file.
So raid_disks is correct.

Signed-off-by: NeilBrown <neilb@suse.de>

Grow.c

@@ -979,7 +979,7 @@ int Grow_reshape(char *devname, int fd, int quiet, char *backup_file,
 			rv = 1;
 			break;
 		}
-		nrdisks = array.nr_disks + sra->array.spare_disks;
+		nrdisks = array.raid_disks + sra->array.spare_disks;
 		/* Now we need to open all these devices so we can read/write.
 		 */
 		fdlist = malloc((1+nrdisks) * sizeof(int));