Renamed mptcp_update_tw_socks for consistency

In struct tcp_sock we have a function pointer (*time_wait) to which
mptcp_update_tw_socks was assigned when the current socket was the meta socket.
For consistency, we changed the name of mptcp_update_tw_socks in mptcp_time_wait
and the former mptcp_time_wait was changed to mptcp_init_tw_sock.

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