swiotlb: clear io_tlb_start and io_tlb_end in swiotlb_exit

Otherwise is_swiotlb_buffer will return false positives when
we first initialize a swiotlb buffer, but then free it because
we have an IOMMU available.

Fixes: 55897af6 ("dma-direct: merge swiotlb_dma_ops into the dma_direct code")
Reported-by: NSibren Vasse <sibren@sibrenvasse.nl>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NSibren Vasse <sibren@sibrenvasse.nl>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

kernel/dma/swiotlb.c

@@ -378,6 +378,8 @@ void __init swiotlb_exit(void)
 		memblock_free_late(io_tlb_start,
 				   PAGE_ALIGN(io_tlb_nslabs << IO_TLB_SHIFT));
 	}
+	io_tlb_start = 0;
+	io_tlb_end = 0;
 	io_tlb_nslabs = 0;
 	max_segment = 0;
 }