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>

drivers/net/Kconfig

@@ -149,7 +149,6 @@ config MACVTAP
 config IPVLAN
     tristate "IP-VLAN support"
     depends on INET
-    depends on IPV6
     depends on NETFILTER
     depends on NET_L3_MASTER_DEV
     ---help---