When raid1 writes, it needs to schedule writes to some number
of devices, and when all writes have completed, the r1_bio
structure that holds it all together must be freed.
However we must make sure not to free it before all devices
have been considered for submitting writes to.

This happens in two places: when submitting a normal write request
and when submiting a write as part of resync.

This patch makes both these places:
  the same
  simpler
  more correct.