ofproto: Avoid using list_size() to compute length of 'pending' list.

ofproto: Avoid using list_size() to compute length of 'pending' list.

Currently this only gets checked for incoming OpenFlow OFPT_FLOW_MOD
messages, so it's hard to imagine it being any kind of bottleneck, but the
NXAST_LEARN action that is soon to be added will be able to create flows
more quickly than we normally expect from a controller.  (On the other
hand, ofproto-dpif, outside of a special testing mode, always completes
operations immediately, so 'pending' will always have length 0.  But this
change still feels right to me for some reason.)