Allow filtering unsupported protocols

[qdeslandes]

r7 and r8 are used to store the L3 protocol ID (e.g. ETH_P_IP) and the L4 protocol ID (e.g. IPPROTO_TCP). During the L3 and L4 parsing logic, if the protocol is not supported, the register is set to 0 to prevent falsely relying on this header later on:

# Parsing L3 header
if (r7 == ETH_P_IP)
    l3_size = sizeof(struct iphdr);
else if (r7 == ETH_P_IPV6)
    l3_size = sizeof(struct ipv6hdr);
else
    r7 = 0

# Parse the L3 header
...

However, such behaviour prevents filtering on non-supported protocol: r7 does not contain the ID of unsupported protocols, so meta.l3_proto only works for IPv4 and IPv6.

Prevent the header parsing functions in stub.c from resetting the protocol ID to 0.