xen-netback: double free on unload

There is a typo here, "i" vs "j", so we would crash on module_exit().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index a0b50ad..130bcb2 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1968,8 +1968,8 @@
 		del_timer_sync(&netbk->net_timer);
 		kthread_stop(netbk->task);
 		for (j = 0; j < MAX_PENDING_REQS; j++) {
-			if (netbk->mmap_pages[i])
-				__free_page(netbk->mmap_pages[i]);
+			if (netbk->mmap_pages[j])
+				__free_page(netbk->mmap_pages[j]);
 		}
 	}