xen/blkback: Change fast_flush_area to xen_blkbk_unmap, and tweak xen_blk_map_seg.

The previous name ('fast_flush_area') had nothing to do with what it
does right now. Changing the names so that the code dealing with
mapping pages in and out of the guest is called xen_blkbk_[map|unmap].

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>