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.comSigned-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>

include/linux/pfn_t.h

@@ -68,7 +68,7 @@ static inline phys_addr_t pfn_t_to_phys(pfn_t pfn)
 
 static inline void *pfn_t_to_virt(pfn_t pfn)
 {
-	if (pfn_t_has_page(pfn))
+	if (pfn_t_has_page(pfn) && !is_device_private_page(pfn_t_to_page(pfn)))
 		return __va(pfn_t_to_phys(pfn));
 	return NULL;
 }