netfilter: nf_tables: hold mutex on netns pre_exit path

[ 3923b1e4406680d57da7e873da77b1683035d83f ]

clean_net() runs in workqueue while walking over the lists, grab mutex.

Fixes: 767d1216bff8 ("netfilter: nftables: fix possible UAF over chains from packet path in netns")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>