Define zero-page offset 0x1e4 as a scratch field, and use it

The relocatable kernel code needs a scratch field for the decompressor
to determine its own location.  It was using a location inside
struct screen_info; reserve a free location and document it as scratch
instead.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>