net/handshake: Add helpers for parsing incoming TLS Alerts

Kernel TLS consumers can replace common TLS Alert parsing code with
these helpers.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/169047942074.5241.13791647439480672048.stgit@oracle-102.nfsv4bat.org


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