[IPV4] fib_trie: Document locking.

Paul E. McKenney writes:

> Those of use who dive into networking only occasionally would much
> appreciate this.  ;-)

No problem here... 

Acked-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> (but trivial)
Signed-off-by: David S. Miller <davem@davemloft.net>