dm-raid: do not include dm-core.h

In commit 4cc96131 ("dm: move request-based code out to dm-rq.[hc]")
we have a note: "DM targets should _never_ include dm-core.h!". And it
is not used in any DM targets except dm-raid now, so let's remove it
from dm-raid for consistency, also use special helpers instead of
accessing dm_table and mapper_device fields directly. This change is
merely a cleanup and should not affect functionality.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>