mm/damon: use 'struct damon_target *' instead of 'void *' in target_valid()

We could use 'struct damon_target *' directly instead of 'void *' in
target_valid() operation to make code simple.

Link: https://lkml.kernel.org/r/1663241621-13293-1-git-send-email-kaixuxia@tencent.com


Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>