block: submit_bh() inadvertently discards barrier flag on a sync write

Reported by Milan Broz <mbroz@redhat.com>, commit 18ce3751

 inadvertently
made submit_bh() discard the barrier bit for a WRITE_SYNC request. Fix
that up.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>