netfilter: nf_nat: Fix memleak in nf_nat_init

When register_pernet_subsys() fails, nf_nat_bysource
should be freed just like when nf_ct_extend_register()
fails.

Fixes: 1cd472bf

 ("netfilter: nf_nat: add nat hook register functions to nf_nat")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>