aufs: 'wh' attribute for RO branch

While the whiteout on the writable branch have its effect
unconditionally (in latter commit), the one on the readonly branch can
have its effect only when this attribute is specified explicitly.
For the branch attributes, refer to the manual in aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/opts.c

@@ -95,6 +95,8 @@ static match_table_t brperm = {
 };
 
 static match_table_t brattr = {
+	/* ro/rr branch */
+	{AuBrRAttr_WH, AUFS_BRRATTR_WH},
 	/* add more later */
 	{0, NULL}
 };

include/uapi/linux/aufs_type.h

@@ -78,12 +78,17 @@ typedef int16_t aufs_bindex_t;
 
 /* branch permissions and attributes */
 #define AUFS_BRPERM_RO		"ro"
+#define AUFS_BRRATTR_WH		"wh"
 
 #define AuBrPerm_RO		(1 << 1)	/* readonly */
 #define AuBrPerm_Mask		AuBrPerm_RO /* re-commit later */
 
+#define AuBrRAttr_WH		(1 << 7)	/* whiteout-able */
+#define AuBrRAttr_Mask		AuBrRAttr_WH
+
 /* the longest combination */
-#define AuBrPermStrSz	sizeof(AUFS_BRPERM_RO)
+#define AuBrPermStrSz	sizeof(AUFS_BRPERM_RO		\
+			       "+" AUFS_BRRATTR_WH)
 
 typedef struct {
 	char a[AuBrPermStrSz];