dm table: remove needless request_queue NULL pointer checks

Since commit ff9ea323

 ("block, bdi: an active gendisk always has a
request_queue associated with it") the request_queue pointer returned
from bdev_get_queue() shall never be NULL.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>