Currently we use interface indeces as zone IDs for link-local and
interface-local scopes, and since we don't have any tool to configure
zone IDs, there is no need to acquire the afdata lock several times per
packet only to read if_index value.
So, now in6_setscope reads zone IDs for interface-local, link-local and
global scopes without a lock.
Sponsored by: Yandex LLC
MFC after: 2 weeks