geneve: Check family when reusing sockets.

When searching for an existing socket to reuse, the address family
is not taken into account - only port number. This means that an
IPv4 socket could be used for IPv6 traffic and vice versa, which
is sure to cause problems when passing packets.

It is not possible to trigger this problem currently because the
only user of Geneve creates just IPv4 sockets. However, that is
likely to change in the near future.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>