gnetworkmonitornetlink: Refactor some code to reduce duplication

This should introduce no functional changes. Factor out some common
code, flip some arguments around to use the more conventional (data,
length) order, and move some memory management calls out of
if-blocks.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793880