gluon-radv-filterd: replace malloc with calloc

Allocation of the router structure for newly detected router
advertisements was done using malloc and selectively clearing some of
the structure fields. However the redirect boolean was never set to 0,
which means that during update_redirect, we would depend on the random
initialization value of the structure.

This means that sometimes ebtables-tiny wouldn't add the correct rules
for the advertised routes.