zram: introduce zram_entry to prepare dedup functionality

Following patch will implement deduplication functionality
in the zram and it requires an indirection layer to manage
the life cycle of zsmalloc handle. To prepare that, this patch
introduces zram_entry which can be used to manage the life-cycle
of zsmalloc handle. Many lines are changed due to rename but
core change is just simple introduction about newly data structure.

Change-Id: Ibf9912397c8c7dbcf1465550bc83a71f904e41c7
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: https://lore.kernel.org/patchwork/patch/787161/


Patch-mainline: linux-kernel@ Thu, 11 May 2017 22:30:21
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
[swatsrid@codeaurora.org: Fix Trivial Merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>