staging: android: ion: Return an ERR_PTR in ion_map_kernel

The expected return value from ion_map_kernel is an ERR_PTR. The error
path for a vmalloc failure currently just returns NULL, triggering
a warning in ion_buffer_kmap_get. Encode the vmalloc failure as an ERR_PTR.

Reported-by: [e-mail]
Signed-off-by: Laura Abbott <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/android/ion/ion_heap.c

···
 	struct page **tmp = pages;
 
 	if (!pages)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	if (buffer->flags & ION_FLAG_CACHED)
 		pgprot = PAGE_KERNEL;