In case of errors, the printed message had the expected and the seen
value inverted.

This patch simply correct the order: first the expected value, then the
one that has been seen.

Fixes: 10d42734 ("selftests: mptcp: userspace: print error details if any")
Cc: stable@vger.kernel.org
Acked-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>