nvme-auth: set explanation code for failure2 msgs

Some error cases were not setting an auth-failure-reason-code-explanation.
This means an AUTH_Failure2 message will be sent with an explanation value
of 0 which is a reserved value.

Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>