soreuseport: test mixed v4/v6 sockets

Test to validate the behavior of SO_REUSEPORT sockets that are
created with both AF_INET and AF_INET6.  See the commit prior to this
for a description of this behavior.

Signed-off-by: Craig Gallek <kraig@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>