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: Gustavo A. R. Silva <gustavoars@kernel.org>
Message-Id: <20200617212930.GA11728@embeddedor>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>