netdevice: add IPv4 fib add/del ops

Add two new ndo ops for IPv4 fib offload support, add and del.  Add uses
modifiy semantics if fib entry already offloaded.  Drivers implementing the new
ndo ops will return err<0 if programming device fails, for example if device's
tables are full.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>