blk-mq: open code __blk_mq_alloc_request in blk_mq_alloc_request_hctx

blk-mq: open code __blk_mq_alloc_request in blk_mq_alloc_request_hctx

blk_mq_alloc_request_hctx is only used for NVMeoF connect commands, so
tailor it to the specific requirements, and don't bother the general
fast path code with its special twinkles.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]
Reviewed-by: Daniel Wagner <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>