slob: remove bigblock tracking

Remove the bigblock lists in favour of using compound pages and going directly
to the page allocator.  Allocation size is stored in page->private, which also
makes ksize more accurate than it previously was.

Saves ~.5K of code, and 12-24 bytes overhead per >= PAGE_SIZE allocation.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>