md/4.1-rc5-fixes

Assorted fixes for new RAID5 stripe-batching functionality.

Unfortunately this functionality was merged a little prematurely.
The necessary testing and code review is now complete (or as
complete as it can be) and to code passes a variety of tests
and looks quite sensible.

Also a fix for some recent locking changes - a race was introduced
which causes a reshape request to sometimes fail.  No data safety issues.
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