nvmet-fc: use zero-sized array and struct_size() in kzalloc()

Update the code to use a zero-sized array instead of a pointer in
structure nvmet_fc_tgt_queue and use struct_size() in kzalloc().

Notice that 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;
	struct boo entry[];
};

instance = kzalloc(sizeof(struct foo) + sizeof(struct boo) * 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: NGustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: NJames Smart <james.smart@broadcom.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>

drivers/nvme/target/fc.c

@@ -128,12 +128,12 @@ struct nvmet_fc_tgt_queue {
 	struct nvmet_cq			nvme_cq;
 	struct nvmet_sq			nvme_sq;
 	struct nvmet_fc_tgt_assoc	*assoc;
-	struct nvmet_fc_fcp_iod		*fod;		/* array of fcp_iods */
 	struct list_head		fod_list;
 	struct list_head		pending_cmd_list;
 	struct list_head		avail_defer_list;
 	struct workqueue_struct		*work_q;
 	struct kref			ref;
+	struct nvmet_fc_fcp_iod		fod[];		/* array of fcp_iods */
 } __aligned(sizeof(unsigned long long));
 
 struct nvmet_fc_tgt_assoc {
@@ -588,9 +588,7 @@ nvmet_fc_alloc_target_queue(struct nvmet_fc_tgt_assoc *assoc,
 	if (qid > NVMET_NR_QUEUES)
 		return NULL;
 
-	queue = kzalloc((sizeof(*queue) +
-				(sizeof(struct nvmet_fc_fcp_iod) * sqsize)),
-				GFP_KERNEL);
+	queue = kzalloc(struct_size(queue, fod, sqsize), GFP_KERNEL);
 	if (!queue)
 		return NULL;
 
@@ -603,7 +601,6 @@ nvmet_fc_alloc_target_queue(struct nvmet_fc_tgt_assoc *assoc,
 	if (!queue->work_q)
 		goto out_a_put;
 
-	queue->fod = (struct nvmet_fc_fcp_iod *)&queue[1];
 	queue->qid = qid;
 	queue->sqsize = sqsize;
 	queue->assoc = assoc;