nexthop: Add support for IPv4 nexthops

Add support for IPv4 nexthops. If nh_family is set to AF_INET, then
NHA_GATEWAY is expected to be an IPv4 address.

Register for netdev events to be notified of admin up/down changes as
well as deletes. A hash table is used to track nexthop per devices to
quickly convert device events to the affected nexthops.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>