vsock/virtio: fix potential unbounded skb queue

virtio_transport_inc_rx_pkt() checks vvs->rx_bytes + len > vvs->buf_alloc.

virtio_transport_recv_enqueue() skips coalescing for packets
with VIRTIO_VSOCK_SEQ_EOM.

If fed with packets with len == 0 and VIRTIO_VSOCK_SEQ_EOM,
a very large number of packets can be queued
because vvs->rx_bytes stays at 0.

Fix this by estimating the skb metadata size:

	(Number of skbs in the queue) * SKB_TRUESIZE(0)

BUG=b/515133258
TEST=presubmit
RELEASE_NOTE=None

Fixes: 077706165717 ("virtio/vsock: don't use skbuff state to account credit")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kevin Berry <kpberry@google.com>
Cc: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: "Eugenio Pérez" <eperezma@redhat.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux.dev
Change-Id: Ia66d128ca2beae4f09e3d453f669fc3405a52aad
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/154283
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Main-Branch-Verified: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
(cherry picked from commit aa1b681f8680ce732fab267651c00239b676697f)
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/154745
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Miri Amarilio <mirilio@google.com>