mptcp: create tcp_sock_ops

Commit 110f35910c75 ("mptcp: MPC flag optimization") introduced additional
function pointers in the struct tcp_sock. This patch groups these callbacks
into a single struct tcp_sock_ops which allows to reduce the size of the
struct tcp_sock.

This patches defines 3 instanciations of this struct, which names are pretty
explicit to what they corresponds: tcp_specific, mptcp_meta_specific and
mptcp_sub_specific.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>