'upcall_xlate()' makes a decision to compose slow path actions
by checking the 'upcall->fitness', which is not initialized in
case of calling from the 'upcall_cb()'.

'upcall_cb()' receives the real flow, so the fitness should be
initialized as perfect.

Fixes following tests on travis:

    ofproto-dpif.at: ofproto-dpif megaflow - disabled - pmd
    ofproto-dpif.at: ofproto-dpif - conntrack - output action

CC: Ben Pfaff <blp@ovn.org>
Fixes: 687bafbb

 ("ofproto-dpif-upcall: Slow path flows that
                      datapath can't fully match.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>