mptcp: Add wrapper around tp->mpc

This allows us to easily use static keys for MPTCP.

Depends-on: db63477e01fa2 (Cleanup)
Reviewed-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: Add static-key check in mptcp()

Static keys allow for less overhead on systems that do not establish
MPTCP connections.

Increments are done when setting mpc to 1 and (at the same time) setting
the sk_destruct-pointer.
Thus, the decrement can be done solely in mptcp_sock_destruct, which is
absolutly necessary as we can only decrement at the very last when the
final MPTCP-socket/subsocket is being destroyed.

Depends-on: c5905afb0ee (static keys: Introduce 'struct static_key', static_key_true()/false() and static_key_slow_[inc|dec]())
Reviewed-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>