UPSTREAM: zsmalloc: add fullness into stat

(cherry-pick from commit 248ca1b053c82fa22427d22b33ac51a24c88a86d)

During investigating compaction, fullness information of each class is
helpful for investigating how the compaction works well.  With that, we
could know how compaction works well more clear on each size class.

Bug: 25951511

Change-Id: I6657a861d05c02bb1dfac4534f6eac7a6ec496d5
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Juneho Choi <juno.choi@lge.com>
Cc: Gunho Lee <gunho.lee@lge.com>
Cc: Luigi Semenzato <semenzato@google.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Jerome Marchand <jmarchan@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>