This implement the locking required by the PM to allow concurent subflow
accessing the msk PM data in a safe way. Such data is protected via a msk-level
spinlock. It relies on the Double checked locking to avoid contention in the
most common scenarion (no or very little PM "actions").

Additionally changes more than a bit the PM hooks/APIs, to be hopefully simpler
and avoiding duplicate code (no v4/v6 variant, work queue management done by the
core PM infra).

It also introduce the PM netlink - it replace the current basic ones, the
related patch has been dropped locally.

git tree available at:

https://github.com/pabeni/mptcp/tree/mptcp_net-next_part3_6

changes from v2:
 - hook the new tests into the self-tests infra

changes from v1:
 - 2 new patches: mp_join self-tests (9/9) and a specific mp_join fix (6/9)
 - fixed several bugs as pointed out by the above tests
 - added support for bind to interface

changes from RFC:
 - cleanup uAPIs
 - added self-tests for the uAPIs
 - fixed ton of bugs
 - included feedback from Mat

 include/uapi/linux/mptcp.h                        |   53 +
 net/mptcp/Makefile                                |    3 
 net/mptcp/options.c                               |   60 -
 net/mptcp/pm.c                                    |  353 +++------
 net/mptcp/pm_netlink.c                            |  828 ++++++++++++++++++++++
 net/mptcp/protocol.c                              |   14 
 net/mptcp/protocol.h                              |  113 +--
 net/mptcp/subflow.c                               |   65 +
 tools/testing/selftests/net/mptcp/Makefile        |    9 
 tools/testing/selftests/net/mptcp/mptcp_connect.c |   19 
 tools/testing/selftests/net/mptcp/mptcp_join.sh   |  296 +++++++
 tools/testing/selftests/net/mptcp/pm_netlink.sh   |  123 +++
 tools/testing/selftests/net/mptcp/pm_nl_ctl.c     |  600 +++++++++++++++
 13 files changed, 2221 insertions(+), 315 deletions(-)