iproute: "list/flush/save default" selected all of the routes

iproute: "list/flush/save default" selected all of the routes

When running "ip route list default" and not specifying address family,
one will get all of the routes instead of just default only. The same
is for "exact default" and "match default".

It behaves in such a way because default route with unspecified family
has the same all-zeroes value like no prefix specified at all. Thus
following code blindly ignores the fact, that prefix was actually
specified.

This patch adds the flag PREFIXLEN_SPECIFIED to the default route too.
And then checks its value when filtering routes.

Signed-off-by: Alexander Zubkov <green@msu.ru>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>