Make SYNCHRONIZE CACHE work with LUNs backed by device files (as opposed

to regular files, which already worked fine).  With this change, it's no
longer neccessary to use "ctladm realsync off" workaround.

sys/cam/ctl/ctl_backend_block.c

@@ -950,6 +950,7 @@ ctl_be_block_cw_dispatch(struct ctl_be_block_lun *be_lun,
 	switch (io->scsiio.cdb[0]) {
 	case SYNCHRONIZE_CACHE:
 	case SYNCHRONIZE_CACHE_16:
+		beio->bio_cmd = BIO_FLUSH;
 		beio->ds_trans_type = DEVSTAT_NO_DATA;
 		beio->ds_tag_type = DEVSTAT_TAG_ORDERED;
 		beio->io_len = 0;