mm/damon/sysfs-schemes: introduce nr_snapshots damos stat file

mm/damon/sysfs-schemes: introduce nr_snapshots damos stat file

Introduce a new DAMON sysfs interface file for exposing the newly added
DAMOS stat, nr_snapshots.  The file has the name same to the stat name
(nr_snapshots) and placed under the damos stat sysfs directory.

Link: https://lkml.kernel.org/r/20251216080128.42991-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>