remoteproc: Harden rproc_handle_vdev() against integer overflow

The struct_size() macro protects against integer overflows but adding
"+ rsc->config_len" introduces the risk of integer overflows again.
Use size_add() to be safe.

Fixes: c8784657 ("remoteproc: use struct_size() helper")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Link: https://lore.kernel.org/r/YyMyoPoGOJUcEpZT@kili


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>