Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>