xen-scsiback: Fix compile warning for 64-bit LUN

drivers/xen/xen-scsiback.c: In function ‘scsiback_add_translation_entry’:
drivers/xen/xen-scsiback.c:911:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 3 has type ‘long long unsigned int’ [-Wformat]

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>