nvme: consolidate memset calls in the nvme_setup_cmd path

In function nvme_setup_cmd() we call command specific setup function
for flush, rw, and discard. Instead of calling memset in each function
lets call it once in the parent function.

This is purely code cleanup patch and it does not change any existing
functionality.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>