flow: Rename skb_mark to pkt_mark.

flow: Rename skb_mark to pkt_mark.

The skb_mark field is currently only available with the Linux datapath
and is only used internally. However, it is desirable to expose this
through OpenFlow and when it is exposed ideally it would not be system-
specific. In preparation for this, skb_mark is rename to pkt_mark in
internal data structures for consistency.

This does not rename the Linux interfaces because doing so would break
the API. It would not necessarily be desirable to do anyways since in
Linux-specific code it is clearer to use the actual name rather than a
generic one. This can lead to confusion in some places, however, because
we do not always strictly separate generic and platform dependent code
(one example is actions). This seems inevitable though at this point if
the lower and upper layers have different names (as they must given the
above requirements).

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>