net/xen-netback: off by one in BUG_ON() condition

The > should be >=.  I also added spaces around the '-' operations so
the code is a little more consistent and matches the condition better.

Fixes: f53c3fe8dad7 ('xen-netback: Introduce TX grant mapping')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -1566,13 +1566,13 @@ static inline void xenvif_tx_dealloc_action(struct xenvif_queue *queue)
 		smp_rmb();
 
 		while (dc != dp) {
-			BUG_ON(gop - queue->tx_unmap_ops > MAX_PENDING_REQS);
+			BUG_ON(gop - queue->tx_unmap_ops >= MAX_PENDING_REQS);
 			pending_idx =
 				queue->dealloc_ring[pending_index(dc++)];
 
-			pending_idx_release[gop-queue->tx_unmap_ops] =
+			pending_idx_release[gop - queue->tx_unmap_ops] =
 				pending_idx;
-			queue->pages_to_unmap[gop-queue->tx_unmap_ops] =
+			queue->pages_to_unmap[gop - queue->tx_unmap_ops] =
 				queue->mmap_pages[pending_idx];
 			gnttab_set_unmap_op(gop,
 					    idx_to_kaddr(queue, pending_idx),