ofctrl: Fix use of uninitialized hash value in ofctrl_add_flow().

ofctrl: Fix use of uninitialized hash value in ofctrl_add_flow().

When ofctrl_add_flow() called ovn_flow_lookup(), the latter used the hash
from the flow's hmap_node, but the former hadn't initialized it at that
point.  This commit fixes the problem.

Reported-by: Russell Bryant <rbryant@redhat.com>
Reported-at: http://openvswitch.org/pipermail/dev/2015-July/057851.html
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>