CVE-2026-43101

In the Linux kernel, the following vulnerability has been resolved:

ipv6: ioam: fix potential NULL dereferences in __ioam6_fill_trace_data()

We need to check __in6_dev_get() for possible NULL value, as
suggested by Yiming Qian.

Also add skb_dst_dev_rcu() instead of skb_dst_dev(),
and two missing READ_ONCE().

Note that @dev can't be NULL.