NTB: Use struct_size() helper in devm_kzalloc()

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes. Also, remove unnecessary
variable _struct_size_.

This code was detected 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]>
Reviewed-by: Logan Gunthorpe <[e-mail]>
Signed-off-by: Jon Mason <[e-mail]>

drivers/ntb/test/ntb_msi_test.c

···
 static int ntb_msit_probe(struct ntb_client *client, struct ntb_dev *ntb)
 {
 	struct ntb_msit_ctx *nm;
-	size_t struct_size;
 	int peers;
 	int ret;
 
···
 		return ret;
 	}
 
-	struct_size = sizeof(*nm) + sizeof(*nm->peers) * peers;
-
-	nm = devm_kzalloc(&ntb->dev, struct_size, GFP_KERNEL);
+	nm = devm_kzalloc(&ntb->dev, struct_size(nm, peers, peers), GFP_KERNEL);
 	if (!nm)
 		return -ENOMEM;