kselftests/damon: add support for cases where debugfs cannot be read

The kernel is in lockdown mode when secureboot is enabled and hence
debugfs cannot be used. Add support for this and other general cases
where debugfs cannot be read and communicate the same to the user before
running tests.

Signed-off-by: Gautam <gautammenghani201@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>