xen-netback: fix sparse warning

Fix warning about 0 used as NULL.
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -1551,7 +1551,7 @@ static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx,
 
 	xenvif_put(vif);
 
-	netbk->mmap_pages[pending_idx]->mapping = 0;
+	netbk->mmap_pages[pending_idx]->mapping = NULL;
 	put_page(netbk->mmap_pages[pending_idx]);
 	netbk->mmap_pages[pending_idx] = NULL;
 }