mm/zsmalloc: allocate exactly size of struct zs_pool

mm/zsmalloc: allocate exactly size of struct zs_pool

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.

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>