block: bypass ->make_request_fn for blk-mq drivers

Call blk_mq_make_request when no ->make_request_fn is set.  This is
safe now that blk_alloc_queue always sets up the pointer for make_request
based drivers.  This avoids an indirect call in the blk-mq driver I/O
fast path, which is rather expensive due to spectre mitigations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>