[ upstream commit b7c4147 ]

The previous commit updating short ICMP error packet NAT
handling improperly computed the length of a short packet
by neglecting to account for the ihl field of the IP header
representing 32 bit words rather than bytes.  This caused
a reversion of #33844.

Furthermore, the change in short packet detection depends on
the incoming bpf_context structure having the "len" field set
correctly, which the BPF tests for that condition did not set.

This corrects the comparison for the inner L4 packet length
and updates the short ICMP error packet BPF tests to ensure
the ctx->len field is set like the kernel does.

Fixes: 1a018d56d623 ("bpf: Refine inner packet L4 checksum detection")

Signed-off-by: Bill Reese <ReeseW@computer.org>
Signed-off-by: Marco Iorio <marco.iorio@isovalent.com>