nvme: pass ubuffer as an integer

This is a prep patch. Modify nvme_submit_user_cmd and
nvme_map_user_request to take ubuffer as plain integer
argument, and do away with nvme_to_user_ptr conversion in callers.

Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220930062749.152261-12-anuj20.g@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>