mm/compaction: make defer_compaction and compaction_deferred static

defer_compaction() and compaction_deferred() and compaction_restarting()
in mm/compaction.c won't be used in other files, so make them static, and
remove the declaration in the header file.

Take the chance to fix a typo.

Link: https://lkml.kernel.org/r/20201123170801.GA9625@rlk
Signed-off-by: Hui Su <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Cc: Nitin Gupta <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Mateusz Nosek <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/compaction.h

···
 extern enum compact_result compaction_suitable(struct zone *zone, int order,
 		unsigned int alloc_flags, int highest_zoneidx);
 
-extern void defer_compaction(struct zone *zone, int order);
-extern bool compaction_deferred(struct zone *zone, int order);
 extern void compaction_defer_reset(struct zone *zone, int order,
 				bool alloc_success);
-extern bool compaction_restarting(struct zone *zone, int order);
 
 /* Compaction has made some progress and retrying makes sense */
 static inline bool compaction_made_progress(enum compact_result result)
···
 					int alloc_flags, int highest_zoneidx)
 {
 	return COMPACT_SKIPPED;
-}
-
-static inline void defer_compaction(struct zone *zone, int order)
-{
-}
-
-static inline bool compaction_deferred(struct zone *zone, int order)
-{
-	return true;
 }
 
 static inline bool compaction_made_progress(enum compact_result result)

mm/compaction.c

···
  * allocation success. 1 << compact_defer_shift, compactions are skipped up
  * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT
  */
-void defer_compaction(struct zone *zone, int order)
+static void defer_compaction(struct zone *zone, int order)
 {
 	zone->compact_considered = 0;
 	zone->compact_defer_shift++;
···
 }
 
 /* Returns true if compaction should be skipped this time */
-bool compaction_deferred(struct zone *zone, int order)
+static bool compaction_deferred(struct zone *zone, int order)
 {
 	unsigned long defer_limit = 1UL << zone->compact_defer_shift;
 
···
 }
 
 /* Returns true if restarting compaction after many failures */
-bool compaction_restarting(struct zone *zone, int order)
+static bool compaction_restarting(struct zone *zone, int order)
 {
 	if (order < zone->compact_order_failed)
 		return false;
···
 }
 
 /*
- * Compound pages of >= pageblock_order should consistenly be skipped until
+ * Compound pages of >= pageblock_order should consistently be skipped until
  * released. It is always pointless to compact pages of such order (if they are
  * migratable), and the pageblocks they occupy cannot contain any free pages.
  */