Revert "gre: Fix IPv6 link-local address generation."

This reverts commit 183185a1.

This patch broke net/forwarding/ip6gre_custom_multipath_hash.sh in some
circumstances (https://lore.kernel.org/netdev/Z9RIyKZDNoka53EO@mini-arch/

).
Let's revert it while the problem is being investigated.

Fixes: 183185a1 ("gre: Fix IPv6 link-local address generation.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/8b1ce738eb15dd841aab9ef888640cab4f6ccfea.1742418408.git.gnault@redhat.com


Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>