hw/net/rocker: Avoid variable length array

hw/net/rocker: Avoid variable length array

Replace an on-stack variable length array in of_dpa_ig() with
a g_autofree heap allocation.

The codebase has very few VLAs, and if we can get rid of them all we
can make the compiler error on new additions.  This is a defensive
measure against security bugs where an on-stack dynamic allocation
isn't correctly size-checked (e.g.  CVE-2021-3527).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>