ipvlan: drop ipv6 dependency

IPVlan has an hard dependency on IPv6, refactor the ipvlan code to allow
compiling it with IPv6 disabled, move duplicate code into addr_equal()
and refactor series of if-else into a switch.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>