net/tun: use reciprocal_scale

Use the inline function reciprocal_scale rather than open coding
the scale optimization.  Also, remove unnecessary initializations.
Resulting compiled code is unchanged (according to godbolt).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20240126002550.169608-1-stephen@networkplumber.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>