BACKPORT: zsmalloc: add more comment

(cherry-pick from commit c3e3e88adccb3119b69484c56798ec616307a94f)

This patch adds lots of comments and it will help others
to review and enhance.

Bug: 25951511

Change-Id: I2c1edf24e917c2d51ef68a9987d81f9b6a4a2bd2
Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>