nvmet: remove unsupported command noise

Nothing can stop a host from submitting invalid commands. The target
just needs to respond with an appropriate status, but that's not a
target error. Demote invalid command messages to the debug level so
these events don't spam the kernel logs.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>