nvme: refactor nvme_submit_user_cmd()

Divide the work into two helpers, namely nvme_alloc_user_request and
nvme_execute_user_rq. This is a prep patch, to help wiring up
uring-cmd support in nvme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
[axboe: fold in fix for assuming bio is non-NULL]
Link: https://lore.kernel.org/r/20220511054750.20432-4-joshi.k@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>