mm/page_alloc.c: change sysctl_lower_zone_reserve_ratio to sysctl_lowmem_reserve_ratio in comments

We use sysctl_lowmem_reserve_ratio rather than
sysctl_lower_zone_reserve_ratio to determine how aggressive the kernel
is in defending lowmem from the possibility of being captured into
pinned user memory.  To avoid misleading, correct it in some comments.
Signed-off-by: NYaowei Bai <bywxiaobai@163.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Acked-by: NDavid Rientjes <rientjes@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -6075,7 +6075,7 @@ void __init page_alloc_init(void)
 }
 
 /*
- * calculate_totalreserve_pages - called when sysctl_lower_zone_reserve_ratio
+ * calculate_totalreserve_pages - called when sysctl_lowmem_reserve_ratio
  *	or min_free_kbytes changes.
  */
 static void calculate_totalreserve_pages(void)
@@ -6119,7 +6119,7 @@ static void calculate_totalreserve_pages(void)
 
 /*
  * setup_per_zone_lowmem_reserve - called whenever
- *	sysctl_lower_zone_reserve_ratio changes.  Ensures that each zone
+ *	sysctl_lowmem_reserve_ratio changes.  Ensures that each zone
  *	has a correct pages reserved value, so an adequate number of
  *	pages are left in the zone after a successful __alloc_pages().
  */