selftests/damon/_damon_sysfs: skip testcases if CONFIG_DAMON_SYSFS is disabled

When CONFIG_DAMON_SYSFS is disabled, the selftests fail with the following
outputs,

not ok 2 selftests: damon: sysfs_update_schemes_tried_regions_wss_estimation.py # exit=1
not ok 3 selftests: damon: damos_quota.py # exit=1
not ok 4 selftests: damon: damos_quota_goal.py # exit=1
not ok 5 selftests: damon: damos_apply_interval.py # exit=1
not ok 6 selftests: damon: damos_tried_regions.py # exit=1
not ok 7 selftests: damon: damon_nr_regions.py # exit=1
not ok 11 selftests: damon: sysfs_update_schemes_tried_regions_hang.py # exit=1

The root cause of this issue is that all the testcases above do not check
the sysfs interface of DAMON whether it exists or not.  With this patch
applied, all the testcases above now pass successfully.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Enze Li <[email protected]>
Reviewed-by: SeongJae Park <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: lienze

tools/testing/selftests/damon/_damon_sysfs.py

@@ -15,6 +15,10 @@
     print('Seems sysfs not mounted?')
     exit(ksft_skip)
 
+if not os.path.exists(sysfs_root):
+    print('Seems DAMON disabled?')
+    exit(ksft_skip)
+
 def write_file(path, string):
     "Returns error string if failed, or None otherwise"
     string = '%s' % string