idiag: provide a hash function for idiag objects

Without ->oo_keygen, libnl will use linear search
for cache objects. This is extremely slow for idiag
when we have a lot of TCP connections. Provide a
hash function for idiag so that libnl will be able
to lookup a hashtable.

http://lists.infradead.org/pipermail/libnl/2014-November/001715.html

Cc: Thomas Graf <tgraf@suug.ch>
Cc: Thomas Haller <thaller@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>