selftests: Set default protocol for raw sockets in nettest

IPPROTO_IP (0) is not valid for raw sockets. Default the protocol for
raw sockets to IPPROTO_RAW if the protocol has not been set via the -P
option.

Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>