selftests/damon/sysfs: hide expected write failures

DAMON selftests for sysfs (sysfs.sh) tests if some writes to DAMON sysfs
interface files fails as expected.  It makes the test results noisy with
the failure error message because it tests a number of such failures. 
Redirect the expected failure error messages to /dev/null to make the
results clean.

Link: https://lkml.kernel.org/r/20230110190400.119388-8-sj@kernel.orgSigned-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>

tools/testing/selftests/damon/sysfs.sh

@@ -24,7 +24,7 @@ ensure_write_fail()
 	content=$2
 	reason=$3
 
-	if echo "$content" > "$file"
+	if (echo "$content" > "$file") 2> /dev/null
 	then
 		echo "writing $content to $file succeed ($fail_reason)"
 		echo "expected failure because $reason"