dsdb: Avoid calculating the PSO multiple times

dsdb: Avoid calculating the PSO multiple times

In a typical user login query, the code tries to work out the PSO 2-3
times - once for the msDS-ResultantPSO attribute, and then again for the
msDS-User-Account-Control-Computed & msDS-UserPasswordExpiryTimeComputed
constructed attributes.

The PSO calculation is reasonably expensive, mostly due to the nested
groups calculation. If we've already constructed the msDS-ResultantPSO
attribute, then we can save ourselves extra work by just re-fetching the
result directly, rather than expanding the nested groups again from
scratch.

The previous patch improves efficiency when there are no PSOs in the
system. This should improve the case where there are PSOs that apply to
the users. (Unfortunately, it won't help where there are some PSOs in
the system, but no PSO applies to the user being queried).

Also updated sam.c so the msDS-ResultantPSO gets calculated first,
before the other constructed attributes.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Wed May 23 10:09:11 CEST 2018 on sn-devel-144