Add Multipath-TCP support.

This patch has been generated from a diff between
https://android.googlesource.com/kernel/samsung/ (branch android-samsung-3.0-jb, commit 3b0c5d2887fca99cab7dd506817b1049d38198a1)
https://github.com/multipath-tcp/mptcp_3.0.x (branch mptcp_samsung, commit 53b77e13702d8d8fe711a075dd1e38e0c8144d5a)