This series bring some fixes and improvement related to the PM netlink,
specifically:

- renames 4th_ack subflow field to 'fully_established':
  this is hopefully more accurate
- implements support to enforce the maximum number of subflows per msk
- implements MP_JOIN 3rd ack retransmission and 4th ack delivery:
  this [ab]use the TCP delack timer. I personally think the idea is nice and
  avoids changes to the core TCP stack, but I guess it could be disputed
- more accurare 'fully established' accounting as per RFC
- drop the msk level 'fully_established' field, as we can use the first subflow
  status without additional locking instead. This simplify core PM event
  handling
- adds more MP_JOIN related mib counters
- update the PM netlink to enforce a configurable maximum number of subflows
  per msk
- extend the pm netlink self-tests to check the above APIs
- uses amost all the above to fix the currently disable mp join self-tests and
 re-enable them

Note: mp join self tests are still unstable, e.g. can fail randomly, due to an
old race between mptcp_token_update_accept() and mptcp_token_get_sock(): e.g.
a client may legitly try to create a subflow after that the first one is fully
established but before the server has accepted the initial socket:
mptcp_token_get_sock() will fail and MP_JOIN will fail too, incrementing
MPTCP_MIB_JOINNOTOKEN.
I think the above should be addressed in a separate series - otherwise this one
will be too big.

The up2date branch for this series (v2) is available here:

https://github.com/pabeni/mptcp/tree/mptcp_net-next_part3_7 

v1 -> v2:
 - comments and readability changes in patch 04

 include/uapi/linux/mptcp.h                      |   23 ++-
 net/mptcp/diag.c                                |    4 
 net/mptcp/mib.c                                 |    3 
 net/mptcp/mib.h                                 |    3 
 net/mptcp/options.c                             |  141 +++++++++++++++++-------
 net/mptcp/pm.c                                  |   35 ++++-
 net/mptcp/pm_netlink.c                          |   93 +++++++++++----
 net/mptcp/protocol.c                            |   23 ++-
 net/mptcp/protocol.h                            |    8 +
 net/mptcp/subflow.c                             |    9 +
 tools/testing/selftests/net/mptcp/mptcp_join.sh |  100 +++++++++++++----
 tools/testing/selftests/net/mptcp/pm_netlink.sh |   17 ++
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c   |   46 ++++---
 13 files changed, 364 insertions(+), 141 deletions(-)