mm/damon/sysfs: link DAMON for virtual address spaces monitoring

This commit links the DAMON sysfs interface to DAMON so that users can
control DAMON via the interface.  In detail, this commit makes writing
'on' to 'state' file constructs DAMON contexts based on values that users
have written to relevant sysfs files and start the context.  It supports
only virtual address spaces monitoring at the moment, though.

The files hierarchy of DAMON sysfs interface after this commit is shown
below.  In the below figure, parents-children relations are represented
with indentations, each directory is having ``/`` suffix, and files in
each directory are separated by comma (",").

    /sys/kernel/mm/damon/admin
    │ kdamonds/nr_kdamonds
    │ │ 0/state,pid
    │ │ │ contexts/nr_contexts
    │ │ │ │ 0/operations
    │ │ │ │ │ monitoring_attrs/
    │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
    │ │ │ │ │ │ nr_regions/min,max
    │ │ │ │ │ targets/nr_targets
    │ │ │ │ │ │ 0/pid_target
    │ │ │ │ │ │ ...
    │ │ │ │ ...
    │ │ ...

The usage is straightforward.  Writing a number ('N') to each 'nr_*' file
makes directories named '0' to 'N-1'.  Users can construct DAMON contexts
by writing proper values to the files in the straightforward manner and
start each kdamond by writing 'on' to 'kdamonds/<N>/state'.

Link: https://lkml.kernel.org/r/20220228081314.5770-5-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Xin Hao <xhao@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>