block: make kblockd_workqueue smarter

kblockd is used for unplugging and may affect IO latency and
throughput and the max number of concurrent work items are bound by
the number of block devices.  Make it HIGHPRI workqueue w/ default max
concurrency.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>