netifd: fix source-routing breaking 6in4 + others

6in4 historically allowed an ip6addr without a mask however the newly
introduced source-routing segfaulted in this scenario (#14958 + #14858).

Fixes include: "Fix ubus route src mask printing" and
"Disable netlink auto ack" (thanks to Hans Dedecker)

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39586 3c298f89-4303-0410-b956-a3cf2f4a3e73