nvme-core: remove unnecessary function parameter

In function nvme_execute_rq() we don't use gendisk parameter at all.
Remove the unsed parameter and adjust the calls.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>