selftests/net: add ipv6 test coverage in rxtimestamp test

Add the options --ipv4, --ipv6 to specify running over ipv4 and/or
ipv6. If neither is specified, then run both.

Signed-off-by: Tanner Love <tannerlove@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>