mptcp: Fullmesh pathmanager: Fix event-handling in case of "multipath on"

If the "multipath on"-flag changes, we schedule a MOD-event. Later when
parsing the sockets we thus will never trigger the creation of new
subflows.

This patch transforms the MOD-event into an ADD-event so that it gets
handled correctly later on.

Reported-by: Kristian Walsh <kristian.walsh@quantumvpn.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>