S390: Update comments about why we don't use <asm-generic/statfs.h>

#ifndef __s390x__

#include <asm-generic/statfs.h>

#else

/*

 * We can't use <asm-generic/statfs.h> because in 64-bit mode

 * we mix ints of different sizes in our struct statfs.

 */

#ifndef __KERNEL_STRICT_NAMES

#include <linux/types.h>

typedef __kernel_fsid_t	fsid_t;

#endif

/*

 * This is ugly -- we're already 64-bit clean, so just duplicate the 

 * definitions.

 */

struct statfs {

	int  f_type;

	int  f_bsize;