From: Mike Christie <michaelc@cs.wisc.edu>

A couple of drivers can sometimes fail the first segments in a bio then
requeue the rest of the request.  In this situation, if the last part of
the bio completes successfully bio_pair_end_* will miss that the beginging
of the bio had failed becuase they just return one when bi_size is not yet
zero.  The attached patch moves the error value test before the bi_size to
catch the above case.