Actually use the 'user_filter' configuration option

config/initializers/devise.rb

@@ -223,6 +223,7 @@ Devise.setup do |config|
       method:   Gitlab.config.ldap['method'],
       bind_dn:  Gitlab.config.ldap['bind_dn'],
       password: Gitlab.config.ldap['password'],
+      filter:   Gitlab.config.ldap['user_filter'],
       name_proc: email_stripping_proc
   end
@@ -244,4 +245,4 @@ Devise.setup do |config|
     config.omniauth provider['name'].to_sym, *provider_arguments
   end
-end
+end
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