ofproto: Avoid extra O(N) work in common case on flow addition.

ofproto: Avoid extra O(N) work in common case on flow addition.

The OpenFlow OFPFF_CHECK_OVERLAP flag requires us to check whether the flow
being inserted overlaps with any existing flows.  That isn't efficiently
implemented and typically requires us to compare the new flow against most
or all of the existing flows.  We don't have to do that work if
OFPFF_CHECK_OVERLAP is not requested, but commit 0b4f207828c (classifier:
Make use of the classifier thread safe.) inadvertently made us do it
anyway.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>