ipv6: Fix ipv6_getsockopt for IPV6_2292PKTOPTIONS

IPV6_2292PKTOPTIONS is broken for 32-bit applications running
in COMPAT mode on 64-bit kernels.

The same problem was fixed for IPv4 with the patch:
ipv4: Fix ip_getsockopt for IP_PKTOPTIONS,
commit dd23198e

Signed-off-by: Sorin Dumitru <sdumitru@ixiacom.com>
Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>