mm: fix spelling error

Fix variable name error in comments. No code changes.

Link: http://lkml.kernel.org/r/20170403161655.5081-1-haolee.swjtu@gmail.com
Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/gfp.h

@@ -313,8 +313,8 @@ static inline bool gfpflags_allow_blocking(const gfp_t gfp_flags)
 
 /*
  * GFP_ZONE_TABLE is a word size bitstring that is used for looking up the
- * zone to use given the lowest 4 bits of gfp_t. Entries are ZONE_SHIFT long
- * and there are 16 of them to cover all possible combinations of
+ * zone to use given the lowest 4 bits of gfp_t. Entries are GFP_ZONES_SHIFT
+ * bits long and there are 16 of them to cover all possible combinations of
  * __GFP_DMA, __GFP_DMA32, __GFP_MOVABLE and __GFP_HIGHMEM.
  *
  * The zone fallback order is MOVABLE=>HIGHMEM=>NORMAL=>DMA32=>DMA.