dm cache metadata: Avoid returning cmd->bm wild pointer on error

commit d16ff19e69ab57e08bf908faaacbceaf660249de upstream.

Maybe __create_persistent_data_objects() caller will use PTR_ERR as a
pointer, it will lead to some strange things.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>