kernel: ipv6: fib6_rules: avoid possible NULL dereference in fib6_rule_action()

In the Linux kernel, the following vulnerability has been resolved: ipv6: fib6rules: avoid possible NULL dereference in fib6ruleaction syzbot is able to trigger the following crash 1, caused by unsafe ip6dstidev use. Indeed ip6dstidev can return NULL, and must always be checked. 1 Oops: general...