selftests/damon/_damon_sysfs: use 'is' instead of '==' for 'None'

_damon_sysfs.py is using '==' or '!=' for 'None'.  Since 'None' is a
singleton, using 'is' or 'is not' is more efficient.  Use the more
efficient one.

Link: https://lkml.kernel.org/r/20240503180318.72798-5-sj@kernel.org


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>