block: pass an explicit gfp_t to get_request

blk_old_get_request already has it at hand, and in blk_queue_bio, which
is the fast path, it is constant.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>