virtio_net: copy zerocopy frags in start_xmit without NAPI

Virtio-net without NAPI frees completed skbs lazily on the next
start_xmit. Senders waiting for in-flight zerocopy buffers can
deadlock if they cannot transmit more packets, as then no
completed packets will be freed.

When !use_napi, virtio-net already calls skb_orphan to avoid waiting
up for transmitted skbs to be freed. For zerocopy packets that
require deep copying on orphan (i.e. those that do not set
SKBFL_DONT_ORPHAN, such as PACKET_TX_RING), call skb_orphan_frags
before orphaning to release the buffers.

This fixes the tpacket_snd slot reuse bug on skb_orphan for
virtio-net, and prevents PACKET_TX_RING from running out of slots.

This fix also touches vhost_net zerocopy packets, which also do not
set SKBFL_DONT_ORPHAN. This is fine: vhost_net packets only encounter
virtio-net in nested virtualization, and only if napi_tx is
explicitly disabled (it has been default-enabled since Linux 4.12).
In that rare case, copying the frags is desirable anyway to prevent
holding guest descriptors pinned across unbounded intervals.

This is a prerequisite for the next patch, which converts
PACKET_TX_RING to standard zerocopy completion. Without this patch
first, a bounded ring sender can stall indefinitely behind a
virtio-net virtqueue that cannot reclaim.

Fixes: 5cd8d46ea156 ("packet: copy user buffers before orphan or clone")
Cc: stable@vger.kernel.org
Cc: mst@redhat.com
Cc: jasowangio@gmail.com
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260919004748.1463985-2-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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