selftests/damon/_damon_sysfs: check errors from nr_schemes file reads

DAMON context staging method in _damon_sysfs.py is not checking the
returned error from nr_schemes file read.  Check it.

Link: https://lkml.kernel.org/r/20240503180318.72798-3-sj@kernel.org
Fixes: f5f0e5a ("selftests/damon/_damon_sysfs: implement kdamonds start function")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sjp38

tools/testing/selftests/damon/_damon_sysfs.py

@@ -341,6 +341,8 @@ def stage(self):
         nr_schemes_file = os.path.join(
                 self.sysfs_dir(), 'schemes', 'nr_schemes')
         content, err = read_file(nr_schemes_file)
+        if err is not None:
+            return err
         if int(content) != len(self.schemes):
             err = write_file(nr_schemes_file, '%d' % len(self.schemes))
             if err != None: