dnsmasq: reset EXTRA_MOUNT in the right place

EXTRA_MOUNT variable should be reset in dnsmasq_start() rather than
just once at the beginning of the script.

Fixes: ac4e8aa2f8 ("dnsmasq: fix more dnsmasq jail issues")
Reported-by: Hartmut Birr <e9hack@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit ddc8d085f3)

package/network/services/dnsmasq/files/dnsmasq.init

@@ -10,7 +10,6 @@ ADD_LOCAL_DOMAIN=1
 ADD_LOCAL_HOSTNAME=1
 ADD_WAN_FQDN=0
 ADD_LOCAL_FQDN=""
-EXTRA_MOUNT=""
 
 BASECONFIGFILE="/var/etc/dnsmasq.conf"
 BASEHOSTFILE="/tmp/hosts/dhcp"
@@ -818,9 +817,10 @@ dnsmasq_start()
 	config_get_bool disabled "$cfg" disabled 0
 	[ "$disabled" -gt 0 ] && return 0
 
-	# reset list of DOMAINS and DNS servers (for each dnsmasq instance)
+	# reset list of DOMAINS, DNS servers and EXTRA mounts (for each dnsmasq instance)
 	DNS_SERVERS=""
 	DOMAIN=""
+	EXTRA_MOUNT=""
 	CONFIGFILE="${BASECONFIGFILE}.${cfg}"
 	CONFIGFILE_TMP="${CONFIGFILE}.$$"
 	HOSTFILE="${BASEHOSTFILE}.${cfg}"