mm/page_owner: add debugfs file 'show_handles'

mm/page_owner: add debugfs file 'show_handles'

Add the flag STACK_PRINT_FLAG_HANDLE to print a stack's handle number from
stackdepot, and add the file 'show_handles' to show just handles and their
number of pages.

This is similar to 'show_stacks', with handles instead of stack traces.

Link: https://lkml.kernel.org/r/20251001175611.575861-4-mfo@igalia.com
Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
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>