ovn-northd: Avoid excessive work to find router ports.

ovn-northd: Avoid excessive work to find router ports.

The ovn_datapath for each logical switch maintains an array of its ports
of type "router-port", but instead of iterating through it build_pre_acls()
iterated through all of the ports in the entire database, which is
wasteful and duplicative work.  This commit switches to using the array of
router ports.

This change is best viewed ignoring white space only changes.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>