netfilter: nf_tables: make table handle allocation per-netns friendly

mutex is per-netns, move table_netns to the pernet area.

*read-write* to 0xffffffff883a01e8 of 8 bytes by task 6542 on cpu 0:
 nf_tables_newtable+0x6dc/0xc00 net/netfilter/nf_tables_api.c:1221
 nfnetlink_rcv_batch net/netfilter/nfnetlink.c:513 [inline]
 nfnetlink_rcv_skb_batch net/netfilter/nfnetlink.c:634 [inline]
 nfnetlink_rcv+0xa6a/0x13a0 net/netfilter/nfnetlink.c:652
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x652/0x730 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x643/0x740 net/netlink/af_netlink.c:1921

Fixes: f102d66b ("netfilter: nf_tables: use dedicated mutex to guard transactions")
Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>