drgn_dump_damon_status: dump damon_target->obsolete

A new field of damon_target for pin-point target removal, namely obsolete,
has newly been added. Extend drgn_dump_damon_status.py to dump it, for
easily writing a future DAMON selftests of it.

Link: https://lkml.kernel.org/r/20251023012535.69625-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Bijan Tabatabai <bijan311@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>