mm/page_owner: add debugfs file 'show_stacks_handles'

mm/page_owner: add debugfs file 'show_stacks_handles'

Add the file 'show_stacks_handles' to show just stack traces and their
handles, in order to resolve stack traces and handles (i.e., to identify
the stack traces for handles in previous reads from 'show_handles').

All stacks/handles must show up, regardless of their number of pages, that
might have become zero or no longer make 'count_threshold', but made it in
previous reads from 'show_handles' -- and need to be resolved later.

P.S.: now, print the extra newline independently of the number of pages.

Link: https://lkml.kernel.org/r/20251001175611.575861-5-mfo@igalia.com
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>