ipv6: Use container_of macro instead of magic number to get ipv6 header.

In ipv6_recv_error(), addr_offset points to daddr field of the ip header.
To get ipv6 header, use container_of() macro instead of substracting magic
number (24).

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>