net/ip6_tunnel: fix dst leak

the commit cdf3464e ("ipv6: Fix dst_entry refcnt bugs in ip6_tunnel")
introduced percpu storage for ip6_tunnel dst cache, but while clearing
such cache it used raw_cpu_ptr to walk the per cpu entries, so cached
dst on non current cpu are not actually reset.

This patch replaces raw_cpu_ptr with per_cpu_ptr, properly cleaning
such storage.

Fixes: cdf3464e ("ipv6: Fix dst_entry refcnt bugs in ip6_tunnel")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>