mm/damon/vaddr: constify static mm_walk_ops

mm/damon/vaddr: constify static mm_walk_ops

The only usage of these structs is to pass their addresses to
walk_page_range(), which takes a pointer to const mm_walk_ops as
argument.  Make them const to allow the compiler to put them in
read-only memory.

Link: https://lkml.kernel.org/r/20211014075042.17174-2-rikard.falkeborn@gmail.com
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>