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/83

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Cc: Matt Porter <[e-mail]>
Cc: Alexandre Bounine <[e-mail]>
Link: http://lkml.kernel.org/r/20200619170843.GA24923@embeddedor
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/rapidio/devices/rio_mport_cdev.c

···
 	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);