fs/proc/task_mmu: fix pagemap flags with PMD THP entries on 32bit

Entries (including flags) are u64, even on 32bit.  So right now we are
cutting of the flags on 32bit.  This way, for example the cow selftest
complains about:

  # ./cow
  ...
  Bail Out! read and ioctl return unmatched results for populated: 0 1

Link: https://lkml.kernel.org/r/20241217195000.1734039-1-david@redhat.com
Fixes: 2c1f057 ("fs/proc/task_mmu: properly detect PM_MMAP_EXCLUSIVE per page of PMD-mapped THPs")
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: davidhildenbrand

fs/proc/task_mmu.c

@@ -1810,7 +1810,7 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
 		}
 
 		for (; addr != end; addr += PAGE_SIZE, idx++) {
-			unsigned long cur_flags = flags;
+			u64 cur_flags = flags;
 			pagemap_entry_t pme;
 
 			if (folio && (flags & PM_PRESENT) &&