tc: Ensure we have enough buffer space when sending filter netlink notifications

tc: Ensure we have enough buffer space when sending filter netlink notifications

The tfilter_notify() and tfilter_del_notify() functions assume that
NLMSG_GOODSIZE is always enough to dump the filter chain. This is not
always the case, which can lead to silent notify failures (because the
return code of tfilter_notify() is not always checked). In particular,
this can lead to NLM_F_ECHO not being honoured even though an action
succeeds, which forces userspace to create workarounds[0].

Fix this by increasing the message size if dumping the filter chain into
the allocated skb fails. Use the size of the incoming skb as a size hint
if set, so we can start at a larger value when appropriate.

To trigger this, run the following commands:

 # ip link add type veth
 # tc qdisc replace dev veth0 root handle 1: fq_codel
 # tc -echo filter add dev veth0 parent 1: u32 match u32 0 0 $(for i in $(seq 32); do echo action pedit munge ip dport set 22; done)

Before this fix, tc just returns:

Not a filter(cmd 2)

After the fix, we get the correct echo:

added filter dev veth0 parent 1: protocol all pref 49152 u32 chain 0 fh 800::800 order 2048 key ht 800 bkt 0 terminal flowid not_in_hw
  match 00000000/00000000 at 0
	action order 1:  pedit action pass keys 1
 	index 1 ref 1 bind 1
	key #0  at 20: val 00000016 mask ffff0000
[repeated 32 times]

[0] https://github.com/openvswitch/ovs/commit/106ef21860c935e5e0017a88bf42b94025c4e511

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Frode Nordahl <frode.nordahl@canonical.com>
Closes: https://bugs.launchpad.net/ubuntu/+source/openvswitch/+bug/2018500
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20250407105542.16601-1-toke@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>