defaults.c: fix ipv6 flow offloading

defaults.c: fix ipv6 flow offloading

The xt_FLOWOFFLOAD target expects that par->thoff points to the beginning
of the TCP header. For IPv4 that is always the case. However, on IPv6, the
field is only initialized, if a protocol match was performed.
Simply matching any protocol (protocol = 0) is enough to trigger the thoff
initialization, so add it to the emitted rule.

Signed-off-by: Felix Fietkau <nbd@nbd.name>