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>