copy_mnt_ns(): use the regular mechanism for freeing empty mnt_ns on failure

Now that free_mnt_ns() works prior to mnt_ns_tree_add(), there's no need for
an open-coded analogue free_mnt_ns() there - yes, we do avoid one call_rcu()
use per failing call of clone() or unshare(), if they fail due to OOM in that
particular spot, but it's not really worth bothering.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/namespace.c

@@ -4184,10 +4184,8 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
 		copy_flags |= CL_SLAVE;
 	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
 	if (IS_ERR(new)) {
+		emptied_ns = new_ns;
 		namespace_unlock();
-		ns_free_inum(&new_ns->ns);
-		dec_mnt_namespaces(new_ns->ucounts);
-		mnt_ns_release(new_ns);
 		return ERR_CAST(new);
 	}
 	if (user_ns != ns->user_ns) {