fib_trie: Move rcu from key_vector to tnode, add accessors.

RCU is only needed once for the entire node, not once per key_vector so we
can pull that out and move it to the tnode structure.

In addition add accessors to be used inside the RCU functions so that we
can more easily get from the key vector to either the tnode or the trie
pointers.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>