blk-mq: initialize req->q in allocation
blk-mq: initialize req->q in allocation

The patch basically reverts the patch of(blk-mq:
initialize request on allocation) in Jens's tree(already
in -next), and only initialize req->q in allocation
for two reasons:

	- presumed cache hotness on completion
	- blk_rq_tagged(rq) depends on reset of req->mq_ctx

Signed-off-by: Ming Lei <572efd9047f12626ad5bcb995a452788646858f3@gmail.com>
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs