VSOCK: handle VIRTIO_VSOCK_OP_CREDIT_REQUEST

The original implementation of the virtio-vsock driver does not
handle a VIRTIO_VSOCK_OP_CREDIT_REQUEST as required by the
virtio-vsock specification. The vsock device emulated by
vhost-vsock and the virtio-vsock driver never uses this request,
which was probably why nobody noticed it. However, another
implementation of the device may use this request type.

Hence, this commit introduces a way to handle an explicit credit
request by responding with a corresponding credit update as
required by the virtio-vsock specification.

Fixes: 06a8fc78

 ("VSOCK: Introduce virtio_vsock_common.ko")
Signed-off-by: Harshavardhan Unnibhavi <harshanavkis@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20210802173506.2383-1-harshanavkis@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>