mm/dev_pfn: exclude MEMORY_DEVICE_PRIVATE while computing virtual address

The presence of struct page does not guarantee linear mapping for the pfn
physical range.  Device private memory which is non-coherent is excluded
from linear mapping during devm_memremap_pages() though they will still
have struct page coverage.

Change pfn_t_to_virt() to just check for device private memory before
giving out virtual address for a given pfn.

pfn_t_to_virt() actually has no callers.  Let's fix it for the 5.2 kernel
and remove it in 5.3.

Link: http://lkml.kernel.org/r/1558089514-25067-1-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>