types.h: define __aligned_u64 and expose to userspace

We currently have a kernel internal type called aligned_u64 which aligns
__u64's on 8 bytes boundaries even on systems which would normally align
them on 4 byte boundaries.  This patch creates a new type __aligned_u64
which does the same thing but which is exposed to userspace rather than
being kernel internal.

[akpm: merge early as both the net and audit trees want this]

[akpm@linux-foundation.org: enhance the comment describing the reasons for using aligned_u64.  Via Andreas and Andi.]
Based-on-patch-by: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: Jan Engelhardt <jengelh@medozas.de>
Cc: David Miller <davem@davemloft.net>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>