[crypto] rework knet_handle_crypto external API to be more solid

the API was rather weak and could potentially leave traffic uncrypted
in case of certain, corner case, failures.

this patch is a subset of a bigger rework of the crypto layer that
will in future allow runtime reconfiguration without traffic disruption
of the crypto config.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>