selftests: mptcp: join: mark tests with data corruption as failed

selftests: mptcp: join: mark tests with data corruption as failed

check_transfer() compares the input and output files byte-by-byte using
`cmp -l "$in" "$out" | while read ...`. Because the while-loop body runs
in a subshell (the script sets neither lastpipe nor pipefail), the
fail_test call inside it -- which sets the global ret/last_test_failed --
and the `return 1` both act on the subshell, not on check_transfer().

check_transfer() thus always falls through to `return 0`, and any data
corruption affecting only the payload (leaving the subflow/PM counters
untouched) is silently reported as PASS.

Fixes: 8117dac3e7c3 ("selftests: mptcp: add invert check in check_transfer")
Cc: stable@vger.kernel.org
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260803-net-mptcp-misc-fixes-7-2-rc6-v2-5-b8f496d71664@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>