netfilter: flowtable: really fix NAT IPv6 offload

The for-loop was broken from the start. It translates to:

	for (i = 0; i < 4; i += 4)

which means the loop statement is run only once, so only the highest
32-bit of the IPv6 address gets mangled.

Fix the loop increment.

Fixes: 0e07e25b ("netfilter: flowtable: fix NAT IPv6 offload mangling")
Fixes: 5c27d8d7 ("netfilter: nf_flow_table_offload: add IPv6 support")
Signed-off-by: Qingfang DENG <dqfext@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>