dm ebs: Fix incorrect checking for REQ_OP_FLUSH

REQ_OP_FLUSH was being treated as a flag, but the operation
part of bio->bi_opf must be treated as a whole. Change to
accessing the operation part via bio_op(bio) and checking
for equality.

Signed-off-by: John Dorminy <jdorminy@redhat.com>
Acked-by: Heinz Mauelshagen <heinzm@redhat.com>
Fixes: d3c7b35c ("dm: add emulated block size target")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>