btrfs-progs: subvolume sync: fix handling of -s option

Setting check interval for subvolume sync resulted
in picking wrong element from argv for it's value:

  $ btrfs subvolume sync -s 10 ./dir
  ERROR: invalid sleep interval ./dir
  $ btrfs subvolume sync ./dir -s 10
  Segmentation fault

Signed-off-by: Marek Rusinowski <marekrusinowski@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

cmds-subvolume.c

@@ -1195,10 +1195,9 @@ static int cmd_subvol_sync(int argc, char **argv)
 
 		switch (c) {
 		case 's':
-			sleep_interval = atoi(argv[optind]);
+			sleep_interval = atoi(optarg);
 			if (sleep_interval < 1) {
-				error("invalid sleep interval %s",
-					argv[optind]);
+				error("invalid sleep interval %s", optarg);
 				ret = 1;
 				goto out;
 			}