md: flush ->event_work before stopping array.

[ Upstream commit ee5d004fd0591536a061451eba2b187092e9127c ]

The 'event_work' worker used by dm-raid may still be running
when the array is stopped.  This can result in an oops.

So flush the workqueue on which it is run after detaching
and before destroying the device.

Reported-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Cc: stable@vger.kernel.org (2.6.38+ please delay 2 weeks after -final release)
Fixes: 9d09e663 ("dm: raid456 basic support")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>