netfilter: synproxy: fix erroneous tcp mss option

Now synproxy sends the mss value set by the user on client syn-ack packet
instead of the mss value that client announced.

Fixes: 48b1de4c

 ("netfilter: add SYNPROXY core/target")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>