Detect packets exceeding MTU

[msune]

Detecting MTU misconfigurations and raise alarms would be a big plus.

Some early thoughts:

• When sfunnel is attached in INGRESS, there is no way to know the egress interface at that point in time.
• Likewise, route MTU checking happens after sfunnel has been executed when attached on INGRESS and before sfunnel is executed when attached on EGRESS. There is literally no way to know that.
• Sidenote: If IPv4 DF=1 is set, Interface counters dropped and/or tx_dropped should increase

What might be more realistic is to define an env variable $DETECT_MAX_MTU=value, and that sfunnel can emit bpf_printk() warnings,with a clear pattern, when funneling exceeds $DETECT_MAX_MTU.