Previously, we called blk_queue_make_request(q,mddev->pers->make_request)
*before* calling mddev->pers->run(), but this left a hole in which the
device could be accessed before it was initialised.

So we moved blk_queue_make_request to *after* ->pers->run(), but now some
of the initialisation done in ->run is over-written by blk_queue_make_request(), 
particularly limits like ->max_sectors.

So now, we just open-code the one line of blk_queue_make_request that we
need at this point.

All the rest has been done by a separate called to blk_queue_make_request, 
either when the mddev was first allocated, or when a previous incarnation
of the device was stopped.

This fixes "bio too big" error that occured due to max_sectors being too large.