Write same support v4

Changes since v3:

 - Document that the REQ_NOMERGE_FLAGS is used for both bio and request
   merging.

 - Do size accounting correctly so we don't have to use
   blk_add_request_payload(). This also fixes an issue discovered by
   Mike: When running over a flaky iSCSI connection we would end up
   completing one block and subsequently fail trying to redrive the
   remainder of the I/O. An unfortunate side effect of the discrepancy
   between the DMA transfer size and the logical block range affected by
   the command.

I put a writesame4 branch on:

   git://git.kernel.org/pub/scm/linux/kernel/git/mkp/linux.git

-- 
Martin K. Petersen	Oracle Linux Engineering

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