sysinfo: Use explicit types in <linux/sysinfo.h>

Change <linux/sysinfo.h> to use explicitly sized types.  Replace
long/unsigned long with __kernel_[u]long_t so that a non-legacy 32-bit
ABI running on a 64-bit kernel can export those as 64-bit types.

Originally-by: H. J. Lu <hjl.tools@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>