Make <asm-generic/statfs.h> suitable for 64-bit platforms.

At the moment, 64-bit platforms (other than Alpha) are all redefining
things for themselves instead of using <asm-generic/statfs.h>.

As is ARM, since it has special requirements w.r.t. padding.

Make <asm-generic/statfs.h> more generic, and they can use it directly.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>