zsmalloc: always keep per-class stats

Always account per-class `zs_size_stat' stats.  This data will help us
make better decisions during compaction.  We are especially interested
in OBJ_ALLOCATED and OBJ_USED, which can tell us if class compaction
will result in any memory gain.

For instance, we know the number of allocated objects in the class, the
number of objects being used (so we also know how many objects are not
used) and the number of objects per-page.  So we can ensure if we have
enough unused objects to form at least one ZS_EMPTY zspage during
compaction.

We calculate this value on per-class basis so we can calculate a total
number of zspages that can be released.  Which is exactly what a
shrinker wants to know.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>