nvme/ioctl: move blk_mq_free_request() out of nvme_map_user_request()

The callers of nvme_map_user_request() (nvme_submit_user_cmd() and
nvme_uring_cmd_io()) allocate the request, so have them free it if
nvme_map_user_request() fails.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>