ipv6: Stop /128 route from disappearing after pmtu update

ipv6: Stop /128 route from disappearing after pmtu update

This patch is mostly from Steffen Klassert <[email protected]>.
I only removed the (rt6->rt6i_dst.plen == 128) check from
ip6_rt_update_pmtu() because the (rt6->rt6i_flags & RTF_CACHE) test
has already implied it.

This patch:
1. Create RTF_CACHE route for /128 non local route
2. After (1), all routes that allow pmtu update should have a RTF_CACHE
   clone.  Hence, stop updating MTU for any non RTF_CACHE route.

Signed-off-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>
Reviewed-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: David S. Miller <[email protected]>