drivers/rapidio/devices/rio_mport_cdev.c: 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.

This issue was found with the help of Coccinelle and, audited and fixed
manually.

Addresses KSPP ID: https://github.com/KSPP/linux/issues/83Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Link: http://lkml.kernel.org/r/20200619170843.GA24923@embeddedorSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rapidio/devices/rio_mport_cdev.c

@@ -1710,8 +1710,7 @@ static int rio_mport_add_riodev(struct mport_cdev_priv *priv,
 	if (rval & RIO_PEF_SWITCH) {
 		rio_mport_read_config_32(mport, destid, hopcount,
 					 RIO_SWP_INFO_CAR, &swpinfo);
-		size += (RIO_GET_TOTAL_PORTS(swpinfo) *
-			 sizeof(rswitch->nextdev[0])) + sizeof(*rswitch);
+		size += struct_size(rswitch, nextdev, RIO_GET_TOTAL_PORTS(swpinfo));
 	}
 
 	rdev = kzalloc(size, GFP_KERNEL);