vsock/virtio: fix accept queue count leak on transport mismatch

virtio_transport_recv_listen() calls sk_acceptq_added() before
vsock_assign_transport(). If vsock_assign_transport() fails or
selects a different transport, the error path returns without
calling sk_acceptq_removed(), permanently incrementing
sk_ack_backlog.

After approximately backlog+1 such failures, sk_acceptq_is_full()
returns true, causing the listener to reject all new connections.

Fix by moving sk_acceptq_added() to after the transport validation,
matching the pattern used by vmci_transport and hyperv_transport.

Fixes: c0cfa2d8a788 ("vsock: add multi-transports support")
Signed-off-by: Dudu Lu <phx0fer@gmail.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260413131409.19022-1-phx0fer@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>