mm/damon/stat: calculate and expose estimated memory bandwidth

mm/damon/stat: calculate and expose estimated memory bandwidth

The raw form of DAMON's monitoring results captures many details of the
information.  However, not every bit of the information is always required
for understanding practical access patterns.  Especially on real world
production systems of high scale time and size, the raw form is difficult
to be aggregated and compared.

Convert the raw monitoring results into a single number metric, namely
estimated memory bandwidth and expose it to users as a read-only
DAMON_STAT parameter.  The metric represents access intensiveness
(hotness) of the system.  It can easily be aggregated and compared for
high level understanding of the access pattern on large systems.

Link: https://lkml.kernel.org/r/20250604183127.13968-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>