nvmet: return proper error code from discovery ctrl

Return NVME_SC_INVALID_FIELD from discovery controller like normal
controller when executing identify or get log page command.

Signed-off-by: Hou Pu <houpu.main@gmail.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>