selftests/damon: skip test if DAMON is running

Testing the DAMON debugfs files while DAMON is running makes no sense,
as any write to the debugfs files will fail.  This commit makes the test
be skipped in this case.

Link: https://lkml.kernel.org/r/20211201150440.1088-8-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>