mm/zsmalloc: adjust order of functions

Currently functions in zsmalloc.c does not arranged in a readable and
reasonable sequence.  With the more and more functions added, we may
meet below inconvenience.  For example:

Current functions:

    void zs_init()
    {
    }

    static void get_maxobj_per_zspage()
    {
    }

Then I want to add a func_1() which is called from zs_init(), and this
new added function func_1() will used get_maxobj_per_zspage() which is
defined below zs_init().

    void func_1()
    {
        get_maxobj_per_zspage()
    }

    void zs_init()
    {
        func_1()
    }

    static void get_maxobj_per_zspage()
    {
    }

This will cause compiling issue. So we must add a declaration:

    static void get_maxobj_per_zspage();

before func_1() if we do not put get_maxobj_per_zspage() before
func_1().

In addition, puting module_[init|exit] functions at the bottom of the
file conforms to our habit.

So, this patch ajusts function sequence as:

    /* helper functions */
    ...
   ...