blk-mq: add a dma mapping iterator

blk-mq: add a dma mapping iterator

blk_rq_map_sg is maze of nested loops.  Untangle it by creating an
iterator that returns [paddr,len] tuples for DMA mapping, and then
implement the DMA logic on top of this.  This not only removes code
at the source level, but also generates nicer binary code:

$ size block/blk-merge.o.*
   text	   data	    bss	    dec	    hex	filename
  10001	    432	      0	  10433	   28c1	block/blk-merge.o.new
  10317	    468	      0	  10785	   2a21	block/blk-merge.o.old

Last but not least it will be used as a building block for a new
DMA mapping helper that doesn't rely on struct scatterlist.

Signed-off-by: Christoph Hellwig <hch@lst.de>