block: blk-rq-qos: guard rq-qos helpers by static key

Even if blk-rq-qos isn't used or configured, dipping into the queue to
fetch ->rq_qos is a noticeable slowdown and visible in profiles. Add an
unlikely static key around blk-rq-qos, to avoid fetching this cacheline
if blk-iolatency or blk-wbt isn't configured or used.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>