MTU is wrong for Pod's eth0 interface when using GRE tunnel

[tnqn]

Describe the bug

While fixing #5868, @hjiajing and I tested more scenarios and found the issue: when using GRE tunnel, the MTU was reduced 38 bytes, however, packets of that size were not forwarded.

I'm not sure if this is broken by OVS kernel change or the tunnel type has never really worked. The problem is that 38 bytes was calculated from 14 outer MAC, 20 outer IP, and 4 GRE header. However, from packet capture, the actual overhead is 14 outer MAC, 20 outer IP, and 8 GRE header (4 standard header + 4 key optional field). We never use GRE key which can be used to carry OVS tun_id value, it seems OVS always add 4 bytes of key to the GRE header even tun_id is not set, leading to 42 bytes of overhead.

Versions:

• Antrea version (Docker image tag). 1.14.1 and before

[tnqn]

#5880 will fix this.