netlink: implement decoding of INET_DIAG_CLASS_ID attribute

netlink: implement decoding of INET_DIAG_CLASS_ID attribute

This is a new netlink attribute introduced by linux kernel commit
v4.14-rc1~130^2~207.

* linux/inet_diag.h (INET_DIAG_CLASS_ID): New enum.
* netlink_inet_diag.c (inet_diag_msg_nla_decoders): Add
INET_DIAG_CLASS_ID.
* xlat/inet_diag_attrs.in: Add INET_DIAG_CLASS_ID.
* xlat/inet_diag_extended_flags.in: Add 1<<(INET_DIAG_CLASS_ID-1).
* tests/nlattr_inet_diag_msg.c (main): Check INET_DIAG_CLASS_ID
decoding.