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

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>