mm: fix show_smap() for zone_device-pmd ranges

mm: fix show_smap() for zone_device-pmd ranges

Attempting to dump /proc/<pid>/smaps for a process with pmd dax mappings
currently results in the following VM_BUG_ONs:

 kernel BUG at mm/huge_memory.c:1105!
 task: ffff88045f16b140 task.stack: ffff88045be14000
 RIP: 0010:[<ffffffff81268f9b>]  [<ffffffff81268f9b>] follow_trans_huge_pmd+0x2cb/0x340
 [..]
 Call Trace:
  [<ffffffff81306030>] smaps_pte_range+0xa0/0x4b0
  [<ffffffff814c2755>] ? vsnprintf+0x255/0x4c0
  [<ffffffff8123c46e>] __walk_page_range+0x1fe/0x4d0
  [<ffffffff8123c8a2>] walk_page_vma+0x62/0x80
  [<ffffffff81307656>] show_smap+0xa6/0x2b0

 kernel BUG at fs/proc/task_mmu.c:585!
 RIP: 0010:[<ffffffff81306469>]  [<ffffffff81306469>] smaps_pte_range+0x499/0x4b0
 Call Trace:
  [<ffffffff814c2795>] ? vsnprintf+0x255/0x4c0
  [<ffffffff8123c46e>] __walk_page_range+0x1fe/0x4d0
  [<ffffffff8123c8a2>] walk_page_vma+0x62/0x80
  [<ffffffff81307696>] show_smap+0xa6/0x2b0

These locations are sanity checking page flags that must be set for an
anonymous transparent huge page, but are not set for the zone_device
pages associated with dax mappings.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>