selftets/damon/sysfs: test existence and permission of avail_operations

This commit adds a selftest test case for ensuring the existence and the
permission (read-only) of the 'avail_oprations' DAMON sysfs file.

Link: https://lkml.kernel.org/r/20220426203843.45238-4-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>