devm_memremap: convert to return ERR_PTR

Make devm_memremap consistent with the error return scheme of
devm_memremap_pages to remove special casing in the pmem driver.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/kernel/memremap.c b/kernel/memremap.c
index 0756273..0d818ce 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -116,7 +116,7 @@
 
 	ptr = devres_alloc(devm_memremap_release, sizeof(*ptr), GFP_KERNEL);
 	if (!ptr)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	addr = memremap(offset, size, flags);
 	if (addr) {