swiotlb: add is_swiotlb_buffer helper function

This adds is_swiotlb_buffer() helper function to see whether a buffer
belongs to the swiotlb buffer or not.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>