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 file backend to reduce the
duplicate code and make the error message uniform.
Signed-off-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>