Clean up struct screen_info (<linux/screen_info.h>)

struct screen_info has unaligned members, it needs to be packed.
In the process, fix the naming of some of the members, which don't
belong in this structure but are part of it anyway.

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