mptcp: Fix Issue #34: Set ECONNRESET on correct socket

sk_it is not initialized at this stage. Rather, ECONNRESET should be set
on the socket we are actually killing (sk).

Reported-by: Eduard Creciun <eduard.creciun@student.uclouvain.be>
Fixes: Bug #34 (https://github.com/multipath-tcp/mptcp/issues/34)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>