iavf: 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:

sizeof(struct virtchnl_ether_addr_list) + (count * sizeof(struct virtchnl_ether_addr))

with:

struct_size(veal, list, count)

and so on...

This code was detected with the help of Coccinelle.

Signed-off-by: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>