UPSTREAM: mm/zsmalloc: avoid duplicate assignment of prev_class

(cherry-pick from commit df8b5bb998f10cfc040ad30300f9a9ea4592ff82)

In zs_create_pool(), prev_class is assigned (ZS_SIZE_CLASSES - 1) times.
And the prev_class only references to the previous size_class.  So we do
not need unnecessary assignement.

This patch assigns *prev_class* when a new size_class structure is
allocated and uses prev_class to check whether the first class has been
allocated.

Bug: 25951511

Change-Id: Ie5e4be867976af0e9ce786a58d1ee0147b7fb0ad
[akpm@linux-foundation.org: remove now-unused ZS_SIZE_CLASSES]
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Reviewed-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>