mm/zsmalloc: add statistics support

Keeping fragmentation of zsmalloc in a low level is our target.  But now
we still need to add the debug code in zsmalloc to get the quantitative
data.

This patch adds a new configuration CONFIG_ZSMALLOC_STAT to enable the
statistics collection for developers.  Currently only the objects
statatitics in each class are collected.  User can get the information via
debugfs.

     cat /sys/kernel/debug/zsmalloc/zram0/...

For example:

After I copied "jdk-8u25-linux-x64.tar.gz" to zram with ext4 filesystem:
 class  size obj_allocated   obj_used pages_used
     0    32             0          0          0
     1    48           256         12          3
     2    64            64         14          1
     3    80            51          7          1
     4    96           128          5          3
     5   112            73          5          2
     6   128            32          4          1
     7   144             0          0          0
     8   160             0         ...