RDMA: Provide safe ib_alloc_device() function

All callers to ib_alloc_device() provide a larger size than struct
ib_device and rely on the fact that struct ib_device is embedded in their
driver specific structure as the first member.

Provide a safer variant of ib_alloc_device() that checks and enforces this
approach to make sure the drivers are using it right.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/rxe/rxe_net.c

@@ -555,7 +555,7 @@ struct rxe_dev *rxe_net_add(struct net_device *ndev)
 	int err;
 	struct rxe_dev *rxe = NULL;
 
-	rxe = (struct rxe_dev *)ib_alloc_device(sizeof(*rxe));
+	rxe = ib_alloc_device(rxe_dev, ib_dev);
 	if (!rxe)
 		return NULL;