CVE-2024-47707 ipv6: avoid possible NULL deref in rt6_uncached_list_flush_dev()

In the Linux kernel, the following vulnerability has been resolved: ipv6: avoid possible NULL deref in rt6uncachedlistflushdev Blamed commit accidentally removed a check for rt-rt6iidev being NULL, as spotted by syzbot: Oops: general protection fault, probably for non-canonical address...