Btrfs: Add mount -o degraded to allow mounts to continue with missing devices

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index 30d1ed2..2bc64fe 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -33,7 +33,8 @@
 int clean_tree_block(struct btrfs_trans_handle *trans,
 		     struct btrfs_root *root, struct extent_buffer *buf);
 struct btrfs_root *open_ctree(struct super_block *sb,
-			      struct btrfs_fs_devices *fs_devices);
+			      struct btrfs_fs_devices *fs_devices,
+			      char *options);
 int close_ctree(struct btrfs_root *root);
 int write_ctree_super(struct btrfs_trans_handle *trans,
 		      struct btrfs_root *root);