mm/damon: s/primitives/code/ on comments

The word 'primitive' is not explicit.  To make the code more easily
understood, this commit renames 'primitives' to 'code' in header comments
of some source files.

Link: https://lkml.kernel.org/r/20250530053115.153238-1-lienze@kylinos.cn


Signed-off-by: Enze Li <lienze@kylinos.cn>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>