dp-packet: Rename 'dp_hash' in 'rss_hash'.

dp-packet: Rename 'dp_hash' in 'rss_hash'.

We already have the 'dp_hash' embedded in the metadata.  This caused
confusion in the code.  With this commit it should be clear that
'rss_hash' is the packet hash used for internal purposes, while
'md.dp_hash' is part of the flow, computed during the execution of
certain actions.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ethan Jackson <ethan@nicira.com>