netfilter: use fwmark_reflect in nf_send_reset

Otherwise, RST packets generated by ipt_REJECT always have mark 0 when
the routing is checked later in the same code path.

Fixes: e110861f ("net: add a sysctl to reflect the fwmark on replies")
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Pau Espin Pedrol <pau.espin@tessares.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>