ip: send ICMP error when DF flag is set and packet is too big

When a packet exceeds the output interface MTU and has the DF (Don't
Fragment) flag set, the router must drop the packet and send an ICMP
"Destination Unreachable" error (Type 3, Code 4: Fragmentation Needed
and DF Set) back to the source.

Previously, grout would silently drop packets that exceeded the MTU
regardless of the DF flag setting. This prevented proper Path MTU
Discovery (RFC 1191) from functioning.

Add a new error node ip_error_frag_needed that generates ICMP Type 3
Code 4 errors using the existing ip_error_process() infrastructure.

Signed-off-by: Anthony Harivel <[email protected]>

Author: aharivel