IB/srp: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding the
size of a structure that has a zero-sized array at the end, along with memory
for some number of elements for that array. For example:

struct foo {
    int stuff;
    void *entry[];
};

instance = kzalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can now
use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/ulp/srp/ib_srp.c

@@ -443,8 +443,7 @@ static struct srp_fr_pool *srp_create_fr_pool(struct ib_device *device,
 	if (pool_size <= 0)
 		goto err;
 	ret = -ENOMEM;
-	pool = kzalloc(sizeof(struct srp_fr_pool) +
-		       pool_size * sizeof(struct srp_fr_desc), GFP_KERNEL);
+	pool = kzalloc(struct_size(pool, desc, pool_size), GFP_KERNEL);
 	if (!pool)
 		goto err;
 	pool->size = pool_size;