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.io


Signed-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>