device-dax/kmem: use struct_size()

Linus notes the kernel has had a nice helper for the 'size of struct with
variable array member at the end' operation for a couple years now, use
it.

Link: http://lore.kernel.org/r/CAHk-=wgNTLbvAD8mNTvh+GQyapNWeX20PXhU_+frqEvVq4298w@mail.gmail.com
Link: https://lkml.kernel.org/r/160288261564.3242821.6055291930923876456.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/dax/kmem.c

@@ -61,7 +61,7 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
 		return -EINVAL;
 	}
 
-	data = kzalloc(sizeof(*data) + sizeof(struct resource *) * dev_dax->nr_range, GFP_KERNEL);
+	data = kzalloc(struct_size(data, res, dev_dax->nr_range), GFP_KERNEL);
 	if (!data)
 		return -ENOMEM;