service/ipfw: Silence warning on restart

Once the ipfw0 interface has been created, ifconfig(8) create will
throw a warning: ifconfig: create: bad value' when trying to create it
again.

PR:		241013
Submitted by:	Jose Luis Duran
Approved by:	kp
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D30083

libexec/rc/rc.d/ipfw

@@ -74,8 +74,14 @@ ipfw_start()
 		${SYSCTL} net.inet.ip.fw.verbose=1 >/dev/null
 	fi
 	if checkyesno firewall_logif; then
-		ifconfig ipfw0 create
-		echo 'Firewall logging pseudo-interface (ipfw0) created.'
+		if ! ifconfig ipfw0 >/dev/null 2>&1; then
+			ifconfig ipfw0 create
+			echo 'Firewall logging pseudo-interface (ipfw0)' \
+			    'created.'
+		else
+			echo 'Firewall logging pseudo-interface (ipfw0)' \
+			    'already created.'
+		fi
 	fi
 }