nvmet: use req->cmd directly in bdev-ns fast path

The function nvmet_bdev_parse_io_cmd() is called from the fast path.
The local variable to that function cmd is only used once.

Remove the local variable and use req->cmd directly.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>