mm: fix struct member name in function comments

The member in struct zonelist is _zonerefs instead of zones.

Link: http://lkml.kernel.org/r/20190927144049.GA29622@haolee.github.ioSigned-off-by: Hao Lee <haolee.swjtu@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -1085,7 +1085,7 @@ static inline struct zoneref *first_zones_zonelist(struct zonelist *zonelist,
 /**
  * for_each_zone_zonelist_nodemask - helper macro to iterate over valid zones in a zonelist at or below a given zone index and within a nodemask
  * @zone - The current zone in the iterator
- * @z - The current pointer within zonelist->zones being iterated
+ * @z - The current pointer within zonelist->_zonerefs being iterated
  * @zlist - The zonelist being iterated
  * @highidx - The zone index of the highest zone to return
  * @nodemask - Nodemask allowed by the allocator