seg6: Fix validation of nexthop addresses

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

seg6: Fix validation of nexthop addresses

The kernel currently validates that the length of the provided nexthop

address does not exceed the specified length. This can lead to the

kernel reading uninitialized memory if user space provided a shorter

length than the specified one.

Fix by validating that the provided length exactly matches the specified

one.