net/sched: cls_api: reclaim an empty proto on the error path

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

net/sched: cls_api: reclaim an empty proto on the error path

Two racing tc filter add requests on the same chain/prio of an
unlocked classifier both run change() on the shared proto and both
can fail: the winner's tcf_chain_tp_delete_empty() attempt gives up
because the loser's handle is still in the idr, and the loser's
error path drops only its own reference without a second reclamation
attempt. The empty proto stays linked in the chain, holding the
chain reference, a block reference and the classifier module
reference until the chain or block is torn down.

Reclaim the proto on the error path of any failed request that holds
a proto reference. The reclamation is emptiness-gated:
tcf_chain_tp_delete_empty() unlinks the proto only when
delete_empty() admits it is empty, so a live shared proto is never
unlinked. A proto the request created is reclaimed unconditionally -
it is the only owner, so marking it for deletion is safe even without
a delete_empty callback. Classifiers without one (the check marks the
proto unconditionally) are rtnl-serialized, so the raced window this
guard closes cannot arise for them.

This is a follow-up to commit d4e359b3608a ("net/sched: cls_api: fix
teardown of an adopted proto on insert-race loss"), which stopped the
loser of the insert race from unlinking the winner's live proto but
left the empty-proto residual in place.

Conditions to recreate:
- CONFIG_NET_CLS_FLOWER=y; veth pair
- tc qdisc add dev veth0 ingress
- two concurrent `tc filter add dev veth0 ingress protocol ip pref 1
flower skip_sw ... action drop` (both fail in fl_hw_replace_filter
after publishing their handle in the idr); repeat in a loop
- an empty flower tp stays linked after both requests fail; visible
as a bare `filter protocol ip pref 1 flower chain 0` header in
`tc filter show` with no filter entries
- CAP_NET_ADMIN (namespace-local via unshare -Urn suffices)