platform: preserve IPv6 multicast route added by kernel
Kernels < 5.11 add a route like:
unicast ff00::/8 dev $IFACE proto boot scope global metric 256 pref medium
to allow sending and receiving IPv6 multicast traffic. Ensure it's not removed it when we do a route sync in mode ALL.
In kernel 5.11 there were commits:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ceed9038b2783d14e0422bdc6fd04f70580efb4c https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a826b04303a40d52439aa141035fca5654ccaccd
After those the route looks like
multicast ff00::/8 dev $IFACE proto kernel metric 256 pref medium
As NM ignores routes with rtm_type multicast, the code in this commit is not needed on newer kernels.