zsmalloc: fixup trivial zs size classes value in comments

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.
Signed-off-by: NWeijie Yang <weijie.yang@samsung.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -141,7 +141,7 @@
 #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
 
 /*
- * On systems with 4K page size, this gives 254 size classes! There is a
+ * On systems with 4K page size, this gives 255 size classes! There is a
  * trader-off here:
  *  - Large number of size classes is potentially wasteful as free page are
  *    spread across these classes