blacklistd: Handle fds that are pointing to routing sockets

If the fd has access to make changes via the routing socket, grant full
permission to make filter changes.

Obtained from: https://github.com/zoulasc/blocklist/commit/1b9475b2c8e0be2b9adc4d88e521ed488ac3c43c

(cherry picked from commit b73612a34259a478c53eab408362d7bf879fefa6)