[IPV4]: Convert FIB Trie to RCU.

* Removes RW-lock
* Proteced read functions uses
  rcu_dereference proteced with rcu_read_lock()
* writing of procted pointer w. rcu_assigen_pointer
* Insert/Replace atomic list_replace_rcu
* A BUG_ON condition removed.in trie_rebalance

With help from Paul E. McKenney.
Signed-off-by: Robert Olsson <Robert.Olsson@data.slu.se>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>