drm/nouveau/mmu: use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(*kind) + sizeof(*kind->data) * mmu->kind_nr;

with:

struct_size(kind, data, mmu->kind_nr)

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>