mm/damon/sysfs: add a file for listing available monitoring ops

DAMON programming interface users can know if specific monitoring ops set
is registered or not using 'damon_is_registered_ops()', but there is no
such method for the user space.  To help the case, this commit adds a new
DAMON sysfs file called 'avail_operations' under each context directory
for listing available monitoring ops.  Reading the file will list each
registered monitoring ops on each line.

Link: https://lkml.kernel.org/r/20220426203843.45238-3-sj@kernel.org


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