nvme-auth: always set valid seq_num in dhchap reply

Currently a seqnum of zero is sent during uni-directional
authentication. The zero value is reserved for the secure channel
feature which is not yet implemented.

Relevant extract from the spec:
The value 0h is used to indicate that bidirectional authentication
is not performed, but a challenge value C2 is carried in order to
generate a pre-shared key (PSK) for subsequent establishment of a
secure channel

Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>