netfilter: nft_flowtable.sh: re-run with random mtu sizes

netfilter: nft_flowtable.sh: re-run with random mtu sizes

Jakub says:
 nft_flowtable.sh is one of the most flake-atious test for netdev CI currently :(

The root cause is two-fold:
1. the failing part of the test is supposed to make sure that ip
   fragments are forwarded for offloaded flows.
   (flowtable has to pass them to classic forward path).
   path mtu discovery for these subtests is disabled.

2. nft_flowtable.sh has two passes.  One with fixed mtus/file size and
  one where link mtus and file sizes are random.

The CI failures all have same pattern:
  re-run with random mtus and file size: -o 27663 -l 4117 -r 10089 -s 54384840
  [..]
  PASS: dscp_egress: dscp packet counters match
  FAIL: file mismatch for ns1 -> ns2

In some cases this error triggers a bit ealier, sometimes in a later
subtest:
  re-run with random mtus and file size: -o 20201 -l 4555 -r 12657 -s 9405856
  [..]
  PASS: dscp_egress: dscp packet counters match
  PASS: dscp_fwd: dscp packet counters match
  2025/08/17 20:37:52 socat[18954] E write(7, 0x560716b96000, 8192): Broken pipe
  FAIL: file mismatch for ns1 -> ns2
  -rw------- 1 root root 9405856 Aug 17 20:36 /tmp/tmp.2n63vlTrQe

But all logs I saw show same scenario:
1. Failing tests have pmtu discovery off (i.e., ip fragmentation)
2. The test file is much larger than first-pass default (2M Byte)
3. peers have much larger MTUs compared to the 'network'.

These errors are very reproducible when re-running the test with
the same commandline arguments.

The timeout became much more prominent with
1d2fbaad7cd8 ("tcp: stronger sk_rcvbuf checks"): reassembled packets
typically have a skb->truesize more than double the skb length.

As that commit is intentional and pmtud-off with
large-tcp-packets-as-fragments is not normal adjust the test to use a
smaller file for the pmtu-off subtests.

While at it, add more information to pass/fail messages and
also run the dscp alteration subtest with pmtu discovery enabled.

Link: https://netdev.bots.linux.dev/contest.html?test=nft-flowtable-sh
Fixes: f84ab634904c ("selftests: netfilter: nft_flowtable.sh: re-run with random mtu sizes")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20250822071330.4168f0db@kernel.org/
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://patch.msgid.link/20250828214918.3385-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>