selftests: mptcp: join: fix AF_INET6 variable

The Fixes commit is a backport renaming a variable, from AF_INET6 to
MPTCP_LIB_AF_INET6.

The commit has been applied without conflicts, except that it missed one
extra variable that was in v6.6, but not in the version linked to the
Fixes commit.

This variable has then been renamed too to avoid these errors:

  LISTENER_CREATED 10.0.2.1:10100     ./mptcp_join.sh: line 2944: [: 2: unary operator expected
  LISTENER_CLOSED  10.0.2.1:10100     ./mptcp_join.sh: line 2944: [: 2: unary operator expected

Fixes: a17d1419 ("selftests: mptcp: declare event macros in mptcp_lib")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>