From: NeilBrown <neilb@cse.unsw.edu.au>

Currently a raid0 superblock is only written when the array is stopped, so a
crash between creation and stop can lose your data.

This patch marks a superblock 'dirty' at creation and forces a dirty
superblock to be written when the array is started.

Previously we would prod the per-array thread at this point, but as it avoids
certain chores when the array is locked, and the array is locked at this
point, that isn't guaranteed to do the right thing.  Instead we prod the
thread whenever the array is unlocked.

Finally, only write the superblock at array stop if it is needed to mark the
array as 'clean'.  raid0 which is never dirty, doesn't need this.