udf: fix default mode and dmode options handling

On x86 (and several other archs) mode_t is defined as "unsigned short"
and comparing unsigned shorts to negative ints is broken (because short
is promoted to int and then compared). Fix it.

Reported-and-tested-by: Laurent Riffard <laurent.riffard@free.fr>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/udf/udf_sb.h b/fs/udf/udf_sb.h
index 5d32c60..158221e 100644
--- a/fs/udf/udf_sb.h
+++ b/fs/udf/udf_sb.h
@@ -48,6 +48,8 @@
 #define UDF_SPARABLE_MAP15		0x1522U
 #define UDF_METADATA_MAP25		0x2511U
 
+#define UDF_INVALID_MODE		((mode_t)-1)
+
 #pragma pack(1) /* XXX(hch): Why?  This file just defines in-core structures */
 
 struct udf_meta_data {