librmnetctl: Add rmnet_fill_newlink_msg() helper

Routing attributes added to RTM_NEWLINK messages during
rtrmnet_ctl_newvnd() and rtrmnet_ctl_changevnd() are nearly the same.
Create a new helper function to handle adding the common attributes for
this type of Netlink messages.

Change-Id: I7ac8ae5c2ca13d5dfa0e036e73d6efd681f22686
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>