From: Angus Sawyer <angus.sawyer@dsl.pipex.com>

When the last device in a raid1 array is failed (or missing) the r1bio
structure can be released (especially on very fast devices) before
make_request has finished using it.

This patch gets and puts an extra reference to the r1_bio around the
submission loop, and uses the status in r1_bio to maintain the request status
if the last refernce is held by make_request.

This is also more correct for write requests, as a write should succeed
if any write succeeded, not only if the last write succceeded.