netfilter request for -stable 4.9.x inclusion

[Pablo Neira Ayuso]

Hi Greg,

Could you enqueue the following patch for -stable 4.9.x?

commit ab6dd1beac7be3c17f8bf3d38bdf29ecb7293f1e
Author: Xin Long <lucien....@gmail.com>
Date:   Thu Aug 10 10:22:24 2017 +0800

    netfilter: check for seqadj ext existence before adding it in 
nf_nat_setup_info

Cc'ing Laura, combining SNAT+DNAT+ftp helper is currently broken with
4.9.x. The patch above cures the issues.

Thanks.