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>