rtnl_link: add basic IFLA_INFO_* handling

rtnl_link: add basic IFLA_INFO_* handling

Dispatch for IFLA_INFO_* attributes should be performed based on the
string provided in a IFLA_INFO_KIND attribute.

* nlattr.h (struct ifla_linkinfo_ctx): New type definition.
* rtnl_link.c (decode_nla_linkinfo_kind): New function.
(ifla_linkinfo_nla_decoders) <IFLA_INFO_KIND>: Use
decode_nla_linkinfo_kind as decoder.
(decode_ifla_linkinfo): Add struct ifla_linkinfo_ctx context, pass it to
the decode_nlattr call as opaque data.