selftests/damon/debugfs_duplicate_context_creation: hide errors from expected file write failures

debugfs_duplicate_context_creation.sh does an invalid file write to ensure
it fails.  Check of the failure is sufficient, so the error message from
the failure only makes the output unnecessarily noisy.  Hide it.

Link: https://lkml.kernel.org/r/20241028233058.283381-5-sj@kernel.org


Fixes: ade38b8c ("selftest/damon: add a test for duplicate context dirs creation")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Andrew Paniakin <apanyaki@amazon.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <davidgow@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>