core: fix matching of default metric value for static routes

core: fix matching of default metric value for static routes

When NM tries to match a generated connection to a persistent one, it
considers also the metric of static routes. However, if the field is
set to -1 (use default value for the device) on the persistent
connection, the comparison will always fail because the generated
connection contains the actual value read from kernel.

To fix the issue, modify check_possible_match() to deal correctly with
-1 and translate it to the expected value for the current device when
performing the comparison.

This allows connections with static routes and default metric to
properly be re-assumed when NM is restarted.

https://bugzilla.redhat.com/show_bug.cgi?id=1302532