selftests: mptcp: join: validate backup in MPJ

commit 935ff5bb8a1cfcdf8e60c8f5c794d0bbbc234437 upstream.

A peer can notify the other one that a subflow has to be treated as
"backup" by two different ways: either by sending a dedicated MP_PRIO
notification, or by setting the backup flag in the MP_JOIN handshake.

The selftests were previously monitoring the former, but not the latter.
This is what is now done here by looking at these new MIB counters when
validating the 'backup' cases:

  MPTcpExtMPJoinSynBackupRx
  MPTcpExtMPJoinSynAckBackupRx

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it will help to validate a new fix for an issue introduced by this
commit ID.

Fixes: 4596a2c1 ("mptcp: allow creating non-backup subflows")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ Conflicts in mptcp_join.sh because the check are done has changed,
  e.g. in commit 03668c65 ("selftests: mptcp: join: rework detailed
  report"), or commit 985de459 ("selftests: mptcp: centralize stats
  dumping"), etc. Adaptations have been made to use the old way, similar
  to what is done just above. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>