xfrm: add x86 CONFIG_COMPAT support

At the moment it is not possible to use the xfrm netlink interface on
x86_64 with a 32bit userland.

The problem exists because a few structures, e.g. struct xfrm_usersa_info,
have different sizes in user/kernelspace (3 byte padding on x86, 7
byte on x86_64) due to different alignment requirements of "u64".

The following patch set aims to resolve this.

The first two patches add necessary CONFIG_COMPAT_NETLINK_MESSAGES
infrastructure to the netlink in/output path.

Patch 3 is a refactoring patch to split functionality (especially
nlmsg allocation and adding data to the nlmsg) in order to
re-use code and ease review.

Patch 4 adds CONFIG_COMPAT_FOR_U64_ALIGNMENT support to xfrm.

Patch 5 sets CMSG_MSG_COMPAT in sock_aio_write(), depending on
is_compat_task().  If patch 5 is deemed too ugly, just pretend it
doesn't exist; userspace can be patched to use sendmsg instead of
write() to avoid this problem.

 include/linux/netlink.h  |    1
 net/netlink/af_netlink.c |   11
 net/socket.c             |    4
 net/xfrm/Kconfig         |    1
 net/xfrm/xfrm_user.c     |  524 ++++++++++++++++++++++++++++++++++++++++++-----
 5 files changed, 488 insertions(+), 53 deletions(-)