s4-dsdb/util: Reorder DSDB_FLAG_* checks

s4-dsdb/util: Reorder DSDB_FLAG_* checks

On good thing about having more clear function interfaces
(and forcing callers to specify clearly what they want)
is that now I can execute following search:
git grep DSDB_FLAG_NEXT_MODULE | wc -l

This showed that DSDB_FLAG_NEXT_MODULE flag is about 6 times
more frequently used than DSDB_FLAG_OWN_MODULE.
So this should reduce branch prediction by six times
in this part of the code, right :)