md: factor out set_in_sync()

Three separate places in md.c check if the number of active
writes is zero and, if so, sets mddev->in_sync.

There are a few differences, but there shouldn't be:
- it is always appropriate to notify the change in
  sysfs_state, and there is no need to do this outside a
  spin-locked region.
- we never need to check ->recovery_cp.  The state of resync
  is not relevant for whether there are any pending writes
  or not (which is what ->in_sync reports).

So create set_in_sync() which does the correct tests and
makes the correct changes, and call this in all three
places.

Any behaviour changes here a minor and cosmetic.

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