datapath: always export priority and skb_mark in netlink message

datapath: always export priority and skb_mark in netlink message

Handling of missing attributes in netlink can be tricky and turns out
to be error prone. The value (savings in netlink bandwidth) does not
seem to be significant enough to justify allowing them. This patch
series make both kernel and userspace always export priority and
skb_mark attribute. There will be follow on patches in the
direction of making all attributes explicit.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>