Make use of the struct_size() and flex_array_size() helpers instead of
open-coded versions, in order to avoid any potential type mistakes or
integer overflows that, in the worse scenario, could lead to heap
overflows.

Link: https://lore.kernel.org/r/20210927225333.GA192634@embeddedor
Link: https://github.com/KSPP/linux/issues/160Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>