ovl: redirect on rename-dir

Current code returns EXDEV when a directory would need to be copied up to
move.  We could copy up the directory tree in this case, but there's
another, simpler solution: point to old lower directory from moved upper
directory.

This is achieved with a "trusted.overlay.redirect" xattr storing the path
relative to the root of the overlay.  After such attribute has been set,
the directory can be moved without further actions required.

This is a backward incompatible feature, old kernels won't be able to
correctly mount an overlay containing redirected directories.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/ovl_entry.h

@@ -13,6 +13,7 @@ struct ovl_config {
 	char *upperdir;
 	char *workdir;
 	bool default_permissions;
+	bool redirect_dir;
 };
 
 /* private information held for overlayfs's superblock */