block: move q->unplug_work initialization

modprobe loop; rmmod loop effectively creates a blk_queue and destroys it
which results in q->unplug_work being canceled without it ever being
initialized.

Therefore, move the initialization of q->unplug_work from
blk_queue_make_request() to blk_alloc_queue*().

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>