Fix checking if tag's domain is available (mastodon#12265)

Pretty much everything about that line was wrong!

app/lib/activitypub/activity/create.rb

@@ -223,7 +223,7 @@ def process_mention(tag)
 
     account = account_from_uri(tag['href'])
     begin
-      return unless DeliveryFailureTracker.available?(@domain) || !account.nil
+      return unless account || DeliveryFailureTracker.available?(tag['href'])
       account = ActivityPub::FetchRemoteAccountService.new.call(tag['href']) if account.nil?
     rescue HTTP::ConnectionError => e
       Rails.logger.info "Fetching account #{tag['href']} failed: #{e}"