xen/grant-table: Use kmalloc_array() in arch_gnttab_valloc()

* A multiplication for the size determination of a memory allocation
  indicated that an array data structure should be processed.
  Thus reuse the corresponding function "kmalloc_array".

  This issue was detected by using the Coccinelle software.

* Replace the specification of a data type by a pointer dereference
  to make the corresponding size determination a bit safer according to
  the Linux coding style convention.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/grant-table.c

@@ -89,7 +89,7 @@ void arch_gnttab_unmap(void *shared, unsigned long nr_gframes)
 
 static int arch_gnttab_valloc(struct gnttab_vm_area *area, unsigned nr_frames)
 {
-	area->ptes = kmalloc(sizeof(pte_t *) * nr_frames, GFP_KERNEL);
+	area->ptes = kmalloc_array(nr_frames, sizeof(*area->ptes), GFP_KERNEL);
 	if (area->ptes == NULL)
 		return -ENOMEM;