Revert "mptcp: Remove unused argument in tcp_select_initial_window"

This reverts commit ffc2c25582fc7ca1ba2a03069d5eb8c459f1dce5.

mptcp_select_initial_window actually needs the sk-pointer.

Fixes: ffc2c25582fc (mptcp: Remove unused argument in tcp_select_initial_window)
Cc: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>