route: add libnl name prefix to global variable link_policy

Public names should be prefixed with a libnl specific
prefix to avoid name collisions. Rename link_policy to
rtln_link_policy.

Related bugs:
  https://github.com/thom311/libnl/issues/52
  https://bugzilla.gnome.org/show_bug.cgi?id=720902
  https://bugs.gentoo.org/show_bug.cgi?id=494986
  https://bugs.gentoo.org/show_bug.cgi?id=494890

Signed-off-by: Thomas Haller <thaller@redhat.com>