nvme: lightnvm: frees wrong cmd structure

When struct nvme_request was introduced, the nvme_nvm_submit_io was
converted to the new interface. The interface moves nvme_nvm_command
data structure into the struct request pdu. On io completion, rq->cmd is
freed, which should have been the dereferenced pdu nvme_request->cmd.

Fixes: d49187e9 "nvme: introduce struct nvme_request"
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>