checkpatch: add __ref as a sparse modifier

Add __ref as a sparse modifier.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -110,7 +110,8 @@ our $Sparse	= qr{
 			__iomem|
 			__must_check|
 			__init_refok|
-			__kprobes
+			__kprobes|
+			__ref
 		}x;
 our $Attribute	= qr{
 			const|