nvmet: use invalid cmd opcode helper

In the NVMeOF block device backend, file backend, and passthru backend
we reject and report the commands if opcode is not handled.

Use the previously introduced helper in the passthru backend to make the
error message uniform.

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