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

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>