ipv6: fix a bad cast in ip6_dst_lookup_tail()

Fix a bug in ip6_dst_lookup_tail(), where typeof(dst) is
"struct dst_entry **", not "struct dst_entry *"

Reported-by: Fengguang Wu <wfg@linux.intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>