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: Sibren Vasse <sibren@sibrenvasse.nl>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Sibren Vasse <sibren@sibrenvasse.nl>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>