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

Currently raid1 uses PAGE_SIZE read/write requests for resync, as it doesn't
know how to honour per-device restrictions.  This patch uses to bio_add_page
to honour those restrictions and ups the limit on request size to 64K.  This
has a measurable impact on rebuild speed (25M/s -> 60M/s)