Btrfs: Add readonly inode flag

This patch adds readonly inode flag support.  A file with this flag
can't be modified, but can be deleted.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 0c0edac..245952c 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -451,9 +451,9 @@
 /*
  * Inode flags
  */
-#define BTRFS_INODE_NODATASUM 		0x1
-#define BTRFS_INODE_NODATACOW 		0x2
-
+#define BTRFS_INODE_NODATASUM		(1 << 0)
+#define BTRFS_INODE_NODATACOW		(1 << 1)
+#define BTRFS_INODE_READONLY		(1 << 2)
 #define btrfs_clear_flag(inode, flag)	(BTRFS_I(inode)->flags &= \
 					 ~BTRFS_INODE_##flag)
 #define btrfs_set_flag(inode, flag)	(BTRFS_I(inode)->flags |= \