FROMGIT: vsock/virtio: Move length check to callers of virtio_vsock_skb_rx_put()

virtio_vsock_skb_rx_put() only calls skb_put() if the length in the
packet header is not zero even though skb_put() handles this case
gracefully.

Remove the functionally redundant check from virtio_vsock_skb_rx_put()
and, on the assumption that this is a worthwhile optimisation for
handling credit messages, augment the existing length checks in
virtio_transport_rx_work() to elide the call for zero-length payloads.
Since the callers all have the length, extend virtio_vsock_skb_rx_put()
to take it as an additional parameter rather than fish it back out of
the packet header.

Note that the vhost code already has similar logic in
vhost_vsock_alloc_skb().

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-4-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit e4b13dce25b41c63716ca51cbc33428245410592
 git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git

 vhost)
Bug: 421244320
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I60ce676b0739ac7bcf8494dec9cb4672dd7b0159