mm/damon/sysfs-schemes: implement filters directory

DAMOS filters are currently supported by only DAMON kernel API.  To expose
the feature to user space, implement a DAMON sysfs directory named
'filters' under each scheme directory.  Please note that this is
implementing only the directory.  Following commits will implement more
files and directories, and finally connect the DAMOS filters feature.

Link: https://lkml.kernel.org/r/20221205230830.144349-6-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>