uapi: convert u64 to __u64 in exported headers

The u64 type is not defined in any exported kernel headers, so trying to
use it will lead to build failures.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>