swiotlb: use map_single instead of swiotlb_map_single in swiotlb_alloc_coherent

We always need swiotlb memory here so address_needs_mapping and
swiotlb_force testings are irrelevant. map_single should be used here
instead of swiotlb_map_single.

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