swiotlb: Make swiotlb bookkeeping functions visible in the header file.

We put the functions dealing with the operations on
the SWIOTLB buffer in the header and make those functions non-static.
And also make the functions exported via EXPORT_SYMBOL_GPL.

See "swiotlb: swiotlb: add swiotlb_tbl_map_single library function" for
full description of patchset.

[v2: swiotlb_sync_single_range_for_* no more. Remove usage.]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: Albert Herranz <albert_herranz@yahoo.es>