samples/damon/mtier: rename to have damon_sample_ prefix

DAMON sample module, mtier has its name 'mtier'.  It could conflict with
future modules, and not very easy to identify it by name.  Use a prefix,
"damon_sample_" for the name.

Note that this could break users if they depend on the old name.  But it
is just a sample, so no such usage is expected, or known.  Even if such
usage exists, updating it for the new name should be straightforward.

Link: https://lkml.kernel.org/r/20250705175000.56259-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>