bgpd: change priority of fs pbr rules

bgpd: change priority of fs pbr rules

two kind of rules are being set from bgp flowspec: ipset based rules,
and ip rule rules. default route rules may have a lower priority than
the other rules ( that do not support default rules). so, if an ipset
rule without fwmark is being requested, then priority is arbitrarily set
to 1. the other case, priority is set to 0.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>