UPSTREAM: mm/zsmalloc: allocate exactly size of struct zs_pool

(cherry-pick from commit 181366561ac1e1a7bc3b91dbe45e7614a2f758b9)

In zs_create_pool(), we allocate memory more then sizeof(struct zs_pool)
  ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);

This patch allocate memory of exactly needed size.

Bug: 25951511

Change-Id: Iae1216f7ab304164ea7d687e0037574d6738c94e
Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>