net/handshake: Add API for sending TLS Closure alerts

This helper sends an alert only if a TLS session was established.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/169047936730.5241.618595693821012638.stgit@oracle-102.nfsv4bat.org


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