nvmet: simplify the nvmet_req_init() interface

Now that a submission queue holds a reference to its completion queue,
there is no need to pass the cq argument to nvmet_req_init(), so remove
it.

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>