netlink: introduce nla_decoder_t to parse netlink attributes data

netlink: introduce nla_decoder_t to parse netlink attributes data

* nlattr.h (nla_decoder_t): New typedef.
(DECL_NLA): New macro.
(decode_nlattr): Add decoders, size and opaque_data argument.
* nlattr.c: (decode_nlattr_with_data): Likewise.
(decode_nla_*): New functions.
* netlink_sock_diag.c (decode_inet_diag_req_compat)
(decode_inet_diag_req_v2, decode_inet_diag_msg)
(decode_netlink_diag_msg, (decode_packet_diag_msg)
(decode_smc_diag_msg, decode_unix_diag_msg): Add decoders,
size and opaque_data arguments. All callers updated.

Co-authored-by: Fabien Siron <fabien.siron@epita.fr>