zsmalloc: add fullness into stat

zsmalloc: add fullness into stat

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.

Signed-off-by: Minchan Kim <[email protected]>
Cc: Juneho Choi <[email protected]>
Cc: Gunho Lee <[email protected]>
Cc: Luigi Semenzato <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Seth Jennings <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Jerome Marchand <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>