selftests: mptcp: join: fix "userspace pm add & remove address"

selftests: mptcp: join: fix "userspace pm add & remove address"

It looks like this test was broken in v6.1 after the backport of commit
48d73f609dcc ("selftests: mptcp: update userspace pm addr tests").

It was not working because the commit ad3493746ebe ("selftests: mptcp:
add test-cases for mixed v4/v6 subflows") is not in v6.1. This commit
changes how the connections are being created in mptcp_join.sh selftest:
with IPv6 support always enabled. But then in v6.1, the server still
create IPv4 only connections, so without the v4-mapped-v6 format with
the "::ffff:" prefix like we have in v6.3.

The modification here adds a support for connections created in v4 as
well so it fixes the issue in v6.1. This patch is not needed for the
selftests in v6.3 because only IPv6 listening sockets are being created.

Fixes: 8f0ba8ec18f5 ("selftests: mptcp: update userspace pm addr tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>