device-dax: use struct_size()

Use the struct_size() helper for the size of a struct with variable
array member at the end, rather than manually calculating it.

Link: https://lkml.kernel.org/r/20211202204422.26777-7-joao.m.martins@oracle.comSuggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/dax/device.c

@@ -404,8 +404,9 @@ int dev_dax_probe(struct dev_dax *dev_dax)
 		return -EINVAL;
 
 	if (!pgmap) {
-		pgmap = devm_kzalloc(dev, sizeof(*pgmap) + sizeof(struct range)
-				* (dev_dax->nr_range - 1), GFP_KERNEL);
+		pgmap = devm_kzalloc(dev,
+                       struct_size(pgmap, ranges, dev_dax->nr_range - 1),
+                       GFP_KERNEL);
 		if (!pgmap)
 			return -ENOMEM;
 		pgmap->nr_range = dev_dax->nr_range;