dm: fix congested_fn for request-based device

We neither assign congested_fn for requested-based blk-mq device nor
implement it correctly. So fix both.

Also, remove incorrect comment from dm_init_normal_md_queue and rename
it to dm_init_congested_fn.

Fixes: 4aa9c692 ("bdi: separate out congested state into a separate struct")
Cc: stable@vger.kernel.org
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>