dpif-netlink: rename linux_flow variable to datapath_flow

In the flow related functions, there's a stack variable called
'linux_flow'. Since this code is not specific to Linux anymore,
in this patch, we rename the variable to 'datpath_flow'.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>