md: close a race with setting mddev->in_sync

If ->in_sync is being set just as md_write_start() is being called,
it is possible that set_in_sync() won't see the elevated
->writes_pending, and md_write_start() won't see the set ->in_sync.

To close this race, re-test ->writes_pending after setting ->in_sync,
and add memory barriers to ensure the increment of ->writes_pending
will be seen by the time of this second test, or the new ->in_sync
will be seen by md_write_start().

Add a spinlock to array_state_show() to ensure this temporary
instability is never visible from userspace.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>