chksum: run 256K benchmark on demand

Fix #17945
ZFS-CI-Type: linux

Author: lzsaver

module/zfs/zfs_chksum.c

@@ -212,6 +212,7 @@ chksum_benchit(chksum_stat_t *cs)
 	chksum_run(cs, abd, ctx, 2, &cs->bs4k);
 	chksum_run(cs, abd, ctx, 3, &cs->bs16k);
 	chksum_run(cs, abd, ctx, 4, &cs->bs64k);
+	chksum_run(cs, abd, ctx, 5, &cs->bs256k);
 	chksum_run(cs, abd, ctx, 6, &cs->bs1m);
 	abd_free(abd);