dm: make flush bios explicitly sync

Commit b685d3d65a ("block: treat REQ_FUA and REQ_PREFLUSH as
synchronous") removed REQ_SYNC flag from WRITE_{FUA|PREFLUSH|...}
definitions.  generic_make_request_checks() however strips REQ_FUA and
REQ_PREFLUSH flags from a bio when the storage doesn't report volatile
write cache and thus write effectively becomes asynchronous which can
lead to performance regressions.

Fix the problem by making sure all bios which are synchronous are
properly marked with REQ_SYNC.

Fixes: b685d3d65a ("block: treat REQ_FUA and REQ_PREFLUSH as synchronous")
Cc: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-raid1.c

@@ -260,7 +260,7 @@ static int mirror_flush(struct dm_target *ti)
 	struct mirror *m;
 	struct dm_io_request io_req = {
 		.bi_op = REQ_OP_WRITE,
-		.bi_op_flags = REQ_PREFLUSH,
+		.bi_op_flags = REQ_PREFLUSH | REQ_SYNC,
 		.mem.type = DM_IO_KMEM,
 		.mem.ptr.addr = NULL,
 		.client = ms->io_client,