ofproto: Use classifer cursor API to collect vlan usage.

ofproto: Use classifer cursor API to collect vlan usage.

This was the only place in OVS code that accessed classifier internal
data structures directly.  Use the classifier cursor API instead, so
that following patches can hide classifier internal data structures.

Note: There seems to be no test case to verify that this vlan usage
collection is implemented correctly.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>