sctp_transport_pl_update() is called when transport update its dst and
pathmtu, instead of stopping the PLPMTUD probe timer, PLPMTUD should
start over and reset the probe timer. Otherwise, the PLPMTUD service
would stop.

Fixes: 92548ec2 ("sctp: add the probe timer in transport for PLPMTUD")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>