ovn: Avoid ARP responder for packets from localnet port

ovn: Avoid ARP responder for packets from localnet port

This is required by next commit that allows lswitch with localnet
port to be attached to multiple chassises. Without this patch, if
an ARP request comes from localnet port, on each chassis there will
be an ARP response, which is not desired.

An new stage ls_in_arp_rsp is introduced for ARP responder before
ls_in_l2_lkup.

Suggested-by: Russell Bryant <russell@ovn.org>
Signed-off-by: Han Zhou <zhouhan@gmail.com>
Acked-by: Russell Bryant <russell@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>