NTB/msi: 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 or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/ntb/msi.c:46:23: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Logan Gunthorpe <[e-mail]>
Signed-off-by: Jon Mason <[e-mail]>

drivers/ntb/msi.c

···
 {
 	phys_addr_t mw_phys_addr;
 	resource_size_t mw_size;
-	size_t struct_size;
 	int peer_widx;
 	int peers;
 	int ret;
···
 	if (peers <= 0)
 		return -EINVAL;
 
-	struct_size = sizeof(*ntb->msi) + sizeof(*ntb->msi->peer_mws) * peers;
-
-	ntb->msi = devm_kzalloc(&ntb->dev, struct_size, GFP_KERNEL);
+	ntb->msi = devm_kzalloc(&ntb->dev, struct_size(ntb->msi, peer_mws, peers),
+				GFP_KERNEL);
 	if (!ntb->msi)
 		return -ENOMEM;