ipv4: Fix ip_getsockopt for IP_PKTOPTIONS

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

This happens because msghdr's msg_flags field is always
set to zero. When running in COMPAT mode this should be
set to MSG_CMSG_COMPAT instead.

Signed-off-by: Tiberiu Szocs-Mihai <tszocs@ixiacom.com>
Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>