Btrfs: very minimal locking

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 3dea757..e32ddff 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -116,6 +116,7 @@
 	fs_info->sb_buffer = sb_buffer;
 	fs_info->sb = sb;
 	mutex_init(&fs_info->trans_mutex);
+	mutex_init(&fs_info->fs_mutex);
 	memset(&fs_info->current_insert, 0, sizeof(fs_info->current_insert));
 	memset(&fs_info->last_insert, 0, sizeof(fs_info->last_insert));