Merge branch 'mptcp-mib-counters'

Matthieu Baerts says:

====================
mptcp: add CurrEstab MIB counter

This MIB counter is similar to the one of TCP -- CurrEstab -- available
in /proc/net/snmp. This is useful to quickly list the number of MPTCP
connections without having to iterate over all of them.

Patch 1 prepares its support by adding new helper functions:

 - MPTCP_DEC_STATS(): similar to MPTCP_INC_STATS(), but this time to
   decrement a counter.

 - mptcp_set_state(): similar to tcp_set_state(), to change the state of
   an MPTCP socket, and to inc/decrement the new counter when needed.

Patch 2 uses mptcp_set_state() instead of directly calling
inet_sk_state_store() to change the state of MPTCP sockets.

Patch 3 and 4 validate the new feature in MPTCP "join" and "diag"
selftests.
====================

Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>