netfilter: xt_hashlimit: dl_seq_stop() fix

If dl_seq_start() memory allocation fails, we crash later in
dl_seq_stop(), trying to kfree(ERR_PTR(-ENOMEM))

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>