zsmalloc: clean up many BUG_ON

There are many BUG_ON in zsmalloc.c which is not recommened so change
them as alternatives.

Normal rule is as follows:

1. avoid BUG_ON if possible. Instead, use VM_BUG_ON or VM_BUG_ON_PAGE

2. use VM_BUG_ON_PAGE if we need to see struct page's fields

3. use those assertion in primitive functions so higher functions can
   rely on the assertion in the primitive function.

4. Don't use assertion if following instruction can trigger Oops

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