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

The function nvmet_file_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>