netlink: add unregister call in cleanup

For protocols that use netlink (generic and route for now), the unint
handler seems to have forgotten to call unregister, which will cause
the assertion the next time the module is loaded.

This patch adds unregister call to netlink_unregister_proto() for those
handlers to avoid bad things happen.

Reviewed-by: melifaro
Fixes: 7e5bf68495cc ("netlink: add netlink support")
Pull-request: https://github.com/freebsd/freebsd-src/pull/781
Signed-off-by: Lin Ma <linma@zju.edu.cn>
(cherry picked from commit 4bdf7f695184fb4312462ad475f3302c9da40f13)