dm-mirror: Support atomic writes

Support atomic writes by setting DM_TARGET_ATOMIC_WRITES for the target
type, and also unmasking REQ_ATOMIC from the submitted bio op flags.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>