netfilter: bridge: really save frag_max_size between PRE and POST_ROUTING

We also need to save/store in forward, else br_parse_ip_options call
will zero frag_max_size as well.

Fixes: 93fdd47e ('bridge: Save frag_max_size between PRE_ROUTING and POST_ROUTING')
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>