UPSTREAM: zsmalloc: fixup trivial zs size classes value in comments

(cherry-pick from commit 7eb52512a977854eca51d9b692c2f3be8a0e5eeb)

According to calculation, ZS_SIZE_CLASSES value is 255 on systems with 4K
page size, not 254.  The old value may forget count the ZS_MIN_ALLOC_SIZE
in.

This patch fixes this trivial issue in the comments.

Bug: 25951511

Change-Id: I7f3039f14a6813bc2e97972b6968ac09d87202ed
Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>