vsock: Update msg_count on read_skb()

Dequeuing via vsock_transport::read_skb() left msg_count outdated, which
then confused SOCK_SEQPACKET recv(). Decrease the counter.

Fixes: 634f1a7 ("vsock: support sockmap")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241013-vsock-fixes-for-redir-v2-3-d6577bbfe742@rbox.co

Author: mmhal

net/vmw_vsock/virtio_transport_common.c

@@ -1723,6 +1723,9 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
 	}
 
 	hdr = virtio_vsock_hdr(skb);
+	if (le32_to_cpu(hdr->flags) & VIRTIO_VSOCK_SEQ_EOM)
+		vvs->msg_count--;
+
 	virtio_transport_dec_rx_pkt(vvs, le32_to_cpu(hdr->len));
 	spin_unlock_bh(&vvs->rx_lock);