mm/compaction: improve comment for compact_memory tunable knob handler

sysctl_compaction_handler() is the handler function for compact_memory
tunable knob under /proc/sys/vm, add the missing knob name to make this
more accurate in comment.

No functional change.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -1708,7 +1708,10 @@ static void compact_nodes(void)
 /* The written value is actually unused, all memory is compacted */
 int sysctl_compact_memory;
 
-/* This is the entry point for compacting all nodes via /proc/sys/vm */
+/*
+ * This is the entry point for compacting all nodes via
+ * /proc/sys/vm/compact_memory
+ */
 int sysctl_compaction_handler(struct ctl_table *table, int write,
 			void __user *buffer, size_t *length, loff_t *ppos)
 {