[PMTUd] only notify PMTUd when pmtud_running and don't force a run to avoid lock fighting

When we request a writelock, we should notify PMTUd to abort and cond_signal only
if PMTUd is running.

Also, don't request an immediate rerun as we cannot guarantee that the scheduling
is favourable to our request and we could end up waiting forever.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>