For each active region corresponding to a bit in the bitmap with have
a 14bit counter (and some flags).
This counts
   number of active writes + bit in the on-disk bitmap + delay-needed.

The "delay-needed" is because we always want a delay before clearing a
bit.  So the number here is normally number of active writes plus 2.
If there have been no writes for a while, we drop to 1.
If still no writes we clear the bit and drop to 0.

So for consistency, when setting bit from the on-disk bitmap or by
request from user-space it is best to set the counter to '2' to start
with.

In particular we might also set the NEEDED_MASK flag at this time, and
in all other cases NEEDED_MASK is only set when the counter is 2 or
more.
Signed-off-by: NeilBrown <neilb@suse.de>