mptcp: make mptcp_alloc_mpcb static

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: is_meta_sk() function pointers optimization

Removed the verification of is_meta_sk() (where possible) that separated MPTCP
meta socket code from ordinary TCP workflow by setting function pointers in
struct tcp_sock to the appropriate value (MPTCP or TCP), according to the
current socket - if it is meta socket or not.

Signed-off-by: Cristina Ciocan <cristina.ciocan@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>