[PATCH] fix SG_IO page leak

[PATCH] fix SG_IO page leak

We cannot always rely on ->biotail remaining untouched. Currently we
leak all the pinned user pages when doing cdda ripping at least, so I
see no way around keeping the bio pointer seperate and passing it back
in for unmap. Alternatively, we could invent a struct blk_map_data and
put it on the stack for passing to both map and unmap.