fix unintentional Sprint of two-argument generateFwmark() call

[eeeeeta]

The setupExternalIPServices() function fails due to blindly calling fmt.Sprint on the return values of generateFwmark(), ignoring the fact that 2 values are returned. This results in errors like the following:

E0614 14:03:47.293710       1 service_endpoints_sync.go:343] Failed to verify and cleanup any mangle table rule to FMWARD the traffic to external IP due to Failed to cleanup iptables command to set up FWMARK due to running [/sbin/iptables -t mangle -C PREROUTING -d {REDACTED} -m tcp -p tcp --dport 80 -j MARK --set-mark 4951 <nil> --wait]: exit status 2: iptables v1.8.3 (legacy): MARK: trailing garbage after value for option "--set-mark".

This patch corrects this issue.

[aauren]

LGTM! Thanks @eeeeeta for the catch.

@murali-reddy I searched through and this appears to be the only instance of generateFwmark that was not handled correctly.