x86/boot: Calculate decompression size during boot not build

Currently z_extract_offset is calculated in boot/compressed/mkpiggy.c.
This doesn't work well because mkpiggy.c doesn't know the details of the
decompressor in use. As a result, it can only make an estimation, which
has risks:

 - output + output_len (VO) could be much bigger than input + input_len
   (ZO). In this case, the decompressed kernel plus relocs could overwrite
   the decompression code while it is running.

 - The head code of ZO could be bigger than z_extract_offset. In this case
   an overwrite could happen when the head code is running to move ZO to
   the end of buffer. Though currently the size of the head code is very
   small it's still a potential risk. Since there is no rule to limit the
   size of the head code of ZO, it runs the risk of suddenly becoming a
   (hard to find) bug.

Instead, this moves the z_extract_offset calculation into header.S, and
makes adjustments to be sure that the above two cases can never happen,
and further corrects the comments describing the calculations.

Since we have (in the previous patch) made ZO always be located against
the end of decompression buffer, z_extract_offset is only used here to
calculate an appropriate buffer size (INIT_SIZE), and is not longer used
elsewhere. As such, it can be removed from voffset.h.

Additionally clean up #if/#else #define to improve readability.

Signed-off-by: Yinghai Lu <[email protected]>
Signed-off-by: Baoquan He <[email protected]>
[ Rewrote the changelog and comments. ]
Signed-off-by: Kees Cook <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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