nvmet: add helper to report invalid opcode

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

Add an helper and use it in block device backend to keep the code
and error message uniform.

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