CVE-2024-42109: netfilter: nf_tables: unconditionally flush pending work before notifier

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nftables: unconditionally flush pending work before notifier

syzbot reports:

KASAN: slab-uaf in nftctxupdate include/net/netfilter/nftables.h:1831 KASAN: slab-uaf in nftcommitrelease net/netfilter/nftablesapi.c:9530 KASAN: slab-uaf int nftablestransdestroywork+0x152b/0x1750 net/netfilter/nftablesapi.c:9597 Read of size 2 at addr ffff88802b0051c4 by task kworker/1:1/45 [..] Workqueue: events nftablestransdestroywork Call Trace: nftctxupdate include/net/netfilter/nftables.h:1831 [inline] nftcommitrelease net/netfilter/nftablesapi.c:9530 [inline] nftablestransdestroywork+0x152b/0x1750 net/netfilter/nftablesapi.c:9597

Problem is that the notifier does a conditional flush, but its possible that the table-to-be-removed is still referenced by transactions being processed by the worker, so we need to flush unconditionally.

We could make the flushwork depend on whether we found a table to delete in nf-next to avoid the flush for most cases.

AFAICS this problem is only exposed in nf-next, with commit e169285f8c56 ("netfilter: nftables: do not store nftctx in transaction objects"), with this commit applied there is an unconditional fetch of table->family which is whats triggering the above splat.