Add ipv4 if addr hash.

These changes are based upon something I brought up the other day, namely that we do a full FIB lookup just to find whether a particular unicast ipv4 address is attached to an interface.

Like ipv6, I added an RCU/RTNL protected hash table and reimplemented __ip_dev_find() in terms of it.

These patches apply both with and without the route cache deletion patch applied, but with the route cache removal applied the udpflood test gets faster by ~13 seconds:

real 2m57.500s
user 0m10.640s
sys 2m46.910s

We can probably use this new table for other things as well.