xen: gntdev: fix build warning

addr is actually a virtual address so use an unsigned long. Fixes:

  CC      drivers/xen/gntdev.o
drivers/xen/gntdev.c: In function 'map_grant_pages':
drivers/xen/gntdev.c:268: warning: cast from pointer to integer of different size

Reduce the scope of the variable at the same time.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>