pfn cannot actually be used unless (!info->feature_persistent), nor is
pfn accessed in get_grant() unless (!info->feature_persistent), but silence
this warning anyway. gcc-4.8
drivers/block/xen-blkfront.c: In function 'do_blkif_request':
drivers/block/xen-blkfront.c:508:20: warning: 'pfn' may be used uninitialized in this function [-Wmaybe-uninitialized]
gnt_list_entry = get_grant(&gref_head, pfn, info);
^
drivers/block/xen-blkfront.c:492:19: note: 'pfn' was declared here
unsigned long pfn;