md/raid5: be more careful about write ordering when reshaping.

When we are reshaping an array, it is very important that we read
the data from a particular sector offset before writing new data
at that offset.

In most cases when growing or shrinking an array we read long before
we even consider writing.  But when restriping an array without
changing it size, there is a small possibility that we might have
some data to available write before the read has happened at the same
location.  This would require some stripes to be in cache already.

To guard against this small possibility, we check, before writing,
that the 'old' stripe at the same location is not in the process of
being read.  And we ensure that we mark all 'source' stripes as such
before allowing new 'destination' stripes to proceed.

Signed-off-by: NeilBrown <neilb@suse.de>