mm/damon: move the implementation of damon_insert_region to damon.h

Usually, inline function is declared static since it should sit between
storage and type.  And implement it in a header file if used by multiple
files.

And this change also fixes compile issue when backport damon to 5.10.

  mm/damon/vaddr.c: In function `damon_va_evenly_split_region':
  ./include/linux/damon.h:425:13: error: inlining failed in call to `always_inline' `damon_insert_region': function body not available
  425 | inline void damon_insert_region(struct damon_region *r,
      | ^~~~~~~~~~~~~~~~~~~
  mm/damon/vaddr.c:86:3: note: called from here
  86 | damon_insert_region(n, r, next, t);
     | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Link: https://lkml.kernel.org/r/20211223085703.6142-1-guoqing.jiang@linux.dev
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Guoqing Jiang

include/linux/damon.h

@@ -451,9 +451,18 @@ static inline struct damon_region *damon_last_region(struct damon_target *t)
 #ifdef CONFIG_DAMON
 
 struct damon_region *damon_new_region(unsigned long start, unsigned long end);
-inline void damon_insert_region(struct damon_region *r,
+
+/*
+ * Add a region between two other regions
+ */
+static inline void damon_insert_region(struct damon_region *r,
 		struct damon_region *prev, struct damon_region *next,
-		struct damon_target *t);
+		struct damon_target *t)
+{
+	__list_add(&r->list, &prev->list, &next->list);
+	t->nr_regions++;
+}
+
 void damon_add_region(struct damon_region *r, struct damon_target *t);
 void damon_destroy_region(struct damon_region *r, struct damon_target *t);
 

mm/damon/core.c

@@ -49,17 +49,6 @@ struct damon_region *damon_new_region(unsigned long start, unsigned long end)
 	return region;
 }
 
-/*
- * Add a region between two other regions
- */
-inline void damon_insert_region(struct damon_region *r,
-		struct damon_region *prev, struct damon_region *next,
-		struct damon_target *t)
-{
-	__list_add(&r->list, &prev->list, &next->list);
-	t->nr_regions++;
-}
-
 void damon_add_region(struct damon_region *r, struct damon_target *t)
 {
 	list_add_tail(&r->list, &t->regions_list);