mm/swapfile.c: use struct_size() in kvzalloc()

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[];
  };

  size = sizeof(struct foo) + count * sizeof(struct boo);
  instance = kvzalloc(size, GFP_KERNEL);

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

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

Notice that, in this case, variable size is not necessary, hence it is
removed.

This code was detected with the help of Coccinelle.

Link: http://lkml.kernel.org/r/20190221154622.GA19599@embeddedorSigned-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -2713,9 +2713,8 @@ static struct swap_info_struct *alloc_swap_info(void)
 	struct swap_info_struct *p;
 	unsigned int type;
 	int i;
-	unsigned int size = sizeof(*p) + nr_node_ids * sizeof(struct plist_node);
 
-	p = kvzalloc(size, GFP_KERNEL);
+	p = kvzalloc(struct_size(p, avail_lists, nr_node_ids), GFP_KERNEL);
 	if (!p)
 		return ERR_PTR(-ENOMEM);