xen-swiotlb: remove xen_io_tlb_start and xen_io_tlb_nslabs

The xen_io_tlb_start and xen_io_tlb_nslabs variables are now only used in
xen_swiotlb_init, so replace them with local variables.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>