kernel: net: ipv6: use-after-free in fib6_rule_suppress due to stale res->rt6 pointer

A use-after-free vulnerability was found in the Linux kernel's IPv6 FIB rule lookup path. In fib6rulesuppress, when a route is suppressed and released via ip6rtputflags, the res-rt6 pointer is not cleared. If no later rule supplies a replacement route, fib6rulelookup returns the stale freed rt6in...