flow: rename nl_to_ functions to flow_put_

It appears to be common in netlink world to use get and put
helper functions for routines.

We already have a set of flow_get_* library calls this renames
the set of functions to create netlink messages flow_put_*

This naming scheme looks better to me at least.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>