netlink: restore the ability to delete PINNED routes

route(8) had such ability before migration to netlink. To be able
to delete PINNED routes we pass RTM_F_FORCE to rib_del_route_px()
when userland uses RTF_PINNED flag with RTM_DELETE command.
This is hackish way for route(8) tool, that third-party software
usually doesn't use.

PR: 279988
Tested by: franco at opnsense org
Discussed with: glebius
Differential Revision: https://reviews.freebsd.org/D46301

(cherry picked from commit 3ce003c8b6151f7764b7934de11c789d6b84ee5c)