iommu/vt-d: 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 code of the following form:

size = sizeof(*info) + level * sizeof(info->path[0]);

with:

size = struct_size(info, path, level);

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>