ICMPv6 sockets don't return EINVAL when sending to IPv4 addresses

**Description**

While writing a PacketImpact test for ICMPv6 sockets, I found a discrepancy with Linux.

When bound to a unicast IPv6 address and sending to an IPv4 address, sendto returns `ENETUNREACH` whereas Linux returns `EINVAL`.

When unbound and sending to an IPv4 address, sendto **does not return an error** whereas Linux returns `EINVAL`.

**Steps to reproduce**

Run test `//third_party/gvisor/test/packetimpact/tests:generic_dgram_socket_send_recv_test` with the if statement linked to this bug removed.