mm/damon/sysfs: support online inputs update

Currently, DAMON sysfs interface doesn't provide a way for adjusting DAMON
input parameters while it is turned on.  Therefore, users who want to
reconfigure DAMON need to stop DAMON and restart.  This means all the
monitoring results that accumulated so far, which could be useful, should
be flushed.  This would be inefficient for many cases.

For an example, let's suppose a sysadmin was running a DAMON-based
Operation Scheme to find memory regions not accessed for more than 5 mins
and page out the regions.  If it turns out the 5 mins threshold was too
long and therefore the sysadmin wants to reduce it to 4 mins, the sysadmin
should turn off DAMON, restart it, and wait for at least 4 more minutes so
that DAMON can find the cold memory regions, even though DAMON was knowing
there are regions that not accessed for 4 mins at the time of shutdown.

This commit makes DAMON sysfs interface to support online DAMON input
parameters updates by adding a new input keyword for the 'state' DAMON
sysfs file, 'commit'.  Writing the keyword to the 'state' file while the
corresponding kdamond is running makes the kdamond to read the sysfs file
values again and update the DAMON context.

Link: https://lkml.kernel.org/r/20220429160606.127307-12-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>