netfilter: nf_tables: hold mutex on netns pre_exit path

commit 3923b1e4 upstream.

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

Fixes: 767d1216 ("netfilter: nftables: fix possible UAF over chains from packet path in netns")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>