[fast-reboot]Avoid stopping masked services during fast-reboot (sonic-net#2335)

#### What I did
During fast-reboot there were warnings for few services
sudo fast-reboot
Warning: The unit file, source configuration file or drop-ins of mux.service changed on disk. Run 'systemctl daemon-reload' to reload units.
Dumping conntrack entries failed
Warning: The unit file, source configuration file or drop-ins of nat.service changed on disk. Run 'systemctl daemon-reload' to reload units.
Warning: The unit file, source configuration file or drop-ins of sflow.service changed on disk. Run 'systemctl daemon-reload' to reload units.
Warning: Stopping docker.service, but it can still be activated by:
docker.socket
Watchdog armed for 180 seconds

This is due to the fact that the services are masked and trying to stop them will throw warning

systemctl is-enabled sflow.service
masked
systemctl stop sflow.service
Warning: The unit file, source configuration file or drop-ins of sflow.service changed on disk. Run 'systemctl daemon-reload' to reload units.


#### How I did it
Added check to skip stopping the services in fast-reboot if the services are masked.


#### How to verify it
Execute fast-reboot with the fix and verify.

Author: dgsudharsan

scripts/fast-reboot

@@ -689,6 +689,12 @@ else
 fi
 
 for service in ${SERVICES_TO_STOP}; do
+    # Skip the masked services
+    state=$(systemctl is-enabled ${service})
+    if [[ $state == "masked" ]]; then
+        continue
+    fi
+
     debug "Stopping ${service} ..."
 
     # TODO: These exceptions for nat, sflow, lldp