ip6_tunnel: use generic for_each_ip_tunnel_rcu macro

ip6_tunnel: use generic for_each_ip_tunnel_rcu macro

Remove the locally defined for_each_ip6_tunnel_rcu macro and use
the generic for_each_ip_tunnel_rcu from linux/if_tunnel.h instead.

This eliminates code duplication and ensures consistency across
the kernel tunnel implementations.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260403084619.4107978-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>