swiotlb: make io_tlb_overflow static

We don't need to export io_tlb_overflow_buffer. I'll remove
io_tlb_overflow_buffer completely in the long term though.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>