Btrfs: very minimal locking

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 96cec63..68cafae 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -231,6 +231,7 @@
 	struct buffer_head *sb_buffer;
 	struct super_block *sb;
 	struct mutex trans_mutex;
+	struct mutex fs_mutex;
 };
 
 /*