ntdb: fix database corruption when transaction doesn't change anything.

ntdb: fix database corruption when transaction doesn't change anything.

ntdb's transaction code has an optimization which tdb's doesnt: it
only writes the parts of blocks whose contents have changed.  This
means we can actually have a transaction which turns out to need no
recovery region.

This breaks the recovery setup logic, which sets the current recovery
size to 0 if there's no recovery area, and assumes that we'll always
create a new recovery area since the recovery will always need > 0
bytes.

In fact, if we really haven't changed anything, we can skip the
transaction commit altogether: since this happens at least once with
Samba, it's worth doing.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>