using rhashtable in inethash

During the Networking Workshop I mentioned converting the inet hash
tables over to rhashtable so that we don't allocate this insanely
large hash table at boot time which goes largely unused.

I took a quick look at this last night and the only thing we really
need is the addition of a set of rhashtable interfaces which use
NULLs lists, as the inet hashtables currently require.

Also, I noticed in the netlink changes this really expensive
synchronize_net() added to netlink_release(), is that _really_
necessary?

That's really expensive and my impression was that such a sync is only
needed during hash table resizing, not when getting rid of objects
that we in an rhashtable.

Thomas?