openvswitch: add missing attribute validation for hash

Add missing attribute validation for OVS_PACKET_ATTR_HASH
to the netlink policy.

Fixes: bd1903b7 ("net: openvswitch: add hash info to upcall")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>