ms_block: convert to blk-mq

Straight forward conversion, room for optimization in how everything
is punted to a work queue. Also looks plenty racy all over the map,
with the state changes. I fixed a bunch of them up while doing the
conversion, but there are surely more.

Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>