xfrm: notify default policy on update

This configuration knob is very sensible, it should be notified when
changing.

Fixes: 2d151d39

 ("xfrm: Add possibility to set the default to block if we have no policy")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>