swiotlb: dump used and total slots when swiotlb buffer is full

So far the kernel only prints the requested size if swiotlb buffer if full.
It is not possible to know whether it is simply an out of buffer, or it is
because swiotlb cannot allocate buffer with the requested size due to
fragmentation.

As 'io_tlb_used' is available since commit 71602fe ("swiotlb: add
debugfs to track swiotlb buffer usage"), both 'io_tlb_used' and
'io_tlb_nslabs' are printed when swiotlb buffer is full.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -533,7 +533,8 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 not_found:
 	spin_unlock_irqrestore(&io_tlb_lock, flags);
 	if (!(attrs & DMA_ATTR_NO_WARN) && printk_ratelimit())
-		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes)\n", size);
+		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes), total %lu, used %lu\n",
+			 size, io_tlb_nslabs, io_tlb_used);
 	return DMA_MAPPING_ERROR;
 found:
 	io_tlb_used += nslots;