Normalise /*FALLTHR{OUGH,U}*/

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Issue #12201

lib/libzutil/zutil_import.c

@@ -1035,6 +1035,7 @@ zpool_read_label(int fd, nvlist_t **config, int *num_labels)
 				case EOPNOTSUPP:
 				case ENOSYS:
 					do_slow = B_TRUE;
+					/* FALLTHROUGH */
 				case 0:
 				default:
 					(void) aio_return(&aiocbs[l]);