mm/damon/sysfs: simplify the judgement whether kdamonds are busy

It is unnecessary to get the number of the running kdamond to judge
whether kdamonds are busy.  Here we can use the
damon_sysfs_kdamond_running() helper and return -EBUSY directly when
finding a running kdamond.  Meanwhile, merging with the judgement that a
kdamond has current sysfs command callback request to make the code more
clear.

Link: https://lkml.kernel.org/r/1662302166-13216-1-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>