md/raid1: simplify alloc_behind_master_bio()

Now that we always always pass an offset of 0 and a size
that matches the bio to alloc_behind_master_bio(),
we can remove the offset/size args and simplify the code.

We could probably remove bio_copy_data_partial() too.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>