[Release 1.20] Add more logging for flannel

[manuelbuil]

Backport: #4549

[VestigeJ]

Validated v1.20.14+k3s1 (6149cba)

$ COMMIT=6149cba2594aa801d15a404c9ebc6b8fb893ffbb
$ k3s -v

k3s version v1.20.14+k3s1 (6149cba2)
go version go1.15.15

$ sudo journalctl -u k3s | grep -i flannel

Jan 03 19:08:07 ip-12-31-2-4.us-north-9.compute.internal k3s[11467]: I0103 19:08:07.513717   11467 flannel.go:92] Determining IP address of default interface
Jan 03 19:08:07 ip-12-31-2-4.us-north-9.compute.internal k3s[11467]: I0103 19:08:07.514003   11467 flannel.go:105] Using interface with name eth0 and address 12.31.2.4
Jan 03 19:08:08 ip-12-31-2-4.us-north-9.compute.internal k3s[11467]: I0103 19:08:08.545430   11467 flannel.go:78] Wrote subnet file to /run/flannel/subnet.env
Jan 03 19:08:08 ip-12-31-2-4.us-north-9.compute.internal k3s[11467]: I0103 19:08:08.545445   11467 flannel.go:82] Running backend.