net/tls: Add TLS Alert definitions

I'm about to add support for kernel handshake API consumers to send
TLS Alerts, so introduce the needed protocol definitions in the new
header tls_prot.h.

This presages support for Closure alerts. Also, support for alerts
is a pre-requite for handling session re-keying, where one peer will
signal the need for a re-key by sending a TLS Alert.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/169047934064.5241.8377890858495063518.stgit@oracle-102.nfsv4bat.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>