gluon-core: check_site.lua: only accept /64 as prefix6

A lot of our code does not actually work with other prefix lengths.

Fixes #1014

package/gluon-core/check_site.lua

@@ -19,7 +19,7 @@ need_string 'timezone'
 
 need_string_array('ntp_servers', false)
 
-need_string_match('prefix6', '^[%x:]+/%d+$')
+need_string_match('prefix6', '^[%x:]+/64$')
 
 
 for _, config in ipairs({'wifi24', 'wifi5'}) do