mm/debug: fix parameter passed to page_mapcount_is_type()

As the comments of page_mapcount_is_type() indicate, the parameter passed
to the function should be one more than page->_mapcount.  However,
page->_mapcount is passed to the function by commit 4ffca5a9 ("mm:
support only one page_type per page") where page_type_has_type() is
replaced by page_mapcount_is_type(), but the parameter isn't adjusted.

Fix the parameter for page_mapcount_is_type() to be (page->__mapcount +
1).  Note that the issue doesn't cause any visible impacts due to the
safety gap introduced by PGTY_mapcount_underflow limit.

[akpm@linux-foundation.org: simplify __dump_folio(), per David]
Link: https://lkml.kernel.org/r/20250321120222.1456770-3-gshan@redhat.com


Fixes: 4ffca5a9 ("mm: support only one page_type per page")
Signed-off-by: Gavin Shan <gshan@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: gehao <gehao@kylinos.cn>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>