xen-blkfront: Silence pfn maybe-uninitialized warning

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;

Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: David Vrabel <[email protected]>
Signed-off-by: Tim Gardner <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Acked-by: Roger Pau Monné <[email protected]>

drivers/block/xen-blkfront.c

@@ -489,7 +489,7 @@ static int blkif_queue_request(struct request *req)
 
 			if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
 			    (i % SEGS_PER_INDIRECT_FRAME == 0)) {
-				unsigned long pfn;
+				unsigned long uninitialized_var(pfn);
 
 				if (segments)
 					kunmap_atomic(segments);