vsock/virtio: avoid queuing packets when intermediate queue is empty

When the driver needs to send new packets to the device, it always
queues the new sk_buffs into an intermediate queue (send_pkt_queue)
and schedules a worker (send_pkt_work) to then queue them into the
virtqueue exposed to the device.

This increases the chance of batching, but also introduces a lot of
latency into the communication. So we can optimize this path by
adding a fast path to be taken when there is no element in the
intermediate queue, there is space available in the virtqueue,
and no other process that is sending packets (tx_lock held).

The following benchmarks were run to check improvements in latency and
throughput. The test bed is a host with Intel i7-10700KF CPU @ 3.80GHz
and L1 guest running on QEMU/KVM with vhost process and all vCPUs
pinned individually to pCPUs.

- Latency
   Tool: Fio version 3.37-56
   Mode: pingpong (h-g-h)
   Test runs: 50
   Runtime-per-test: 50s
   Type: SOCK_STREAM

In the following fio benchmark (pingpong mode) the host sends
a payload to the guest and waits for the same payload back.

fio process pinned both inside the host and the guest system.

Before: Linux 6.9.8

Payload 64B:

	1st perc.	overall		99th perc.
Before	12.91		16.78		42.24		us
After	9.77		13.57		39.17		us

Payload 512B:

	1st perc.	overall		99th perc.
Before	13.35		17.35		41.52		us
After	10.25		14.11		39.58		us

Payload 4K:

	1st perc.	overall		99th perc.
Before	14.71		19.87		41.52		us
After	10.51		14.96		40.81		us

- Throughput
   Tool: iperf-vsock

The size represents the buffer length (-l) to read/write
P represents the number of parallel streams

P=1
	4K	64K	128K
Before	6.87	29.3	29.5 Gb/s
After	10.5	39.4	39.9 Gb/s

P=2
	4K	64K	128K
Before	10.5	32.8	33.2 Gb/s
After	17.8	47.7	48.5 Gb/s

P=4
	4K	64K	128K
Before	12.7	33.6	34.2 Gb/s
After	16.9	48.1	50.5 Gb/s

The performance improvement is related to this optimization,
I used a ebpf kretprobe on virtio_transport_send_skb to check
that each packet was sent directly to the virtqueue

Co-developed-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Marco Pinna <marco.pinn95@gmail.com>
Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Message-Id: <20240730-pinna-v4-2-5c9179164db5@outlook.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

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