datapath: Restructure datapath.c and flow.c

datapath: Restructure datapath.c and flow.c

Over the time datapath.c and flow.c has became pretty large files.
Following patch restructures functionality of component into three
different components:

flow.c: contains flow extract.
flow_netlink.c: netlink flow api.
flow_table.c: flow table api.

Diffstat is showing wrong count. This patch mostly restructures code
without changing logic.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>