From: NeilBrown <neilb@cse.unsw.edu.au>

For each resync request, we allocate a "r1_bio" which has a bio "master_bio"
attached that goes largely unused.  We also allocate a read_bio which is
used.  This patch removes the read_bio and just uses the master_bio instead.

This fixes a bug wherein bi_bdev of the master_bio wasn't being set, but was
being used.

We also introduce a new "sectors" field into the r1_bio as we can no-longer
rely in master_bio->bi_sectors.