Extract method to get the tenant root storage

This way we simplify the code a little bit and we create a method unique
to the `TenantDiskService` class, which can be used to check whether
we're using this class without using `is_a?` or similar.

app/lib/active_storage/service/tenant_disk_service.rb

@@ -2,11 +2,19 @@ require "active_storage/service/disk_service"
 
 module ActiveStorage
   class Service::TenantDiskService < Service::DiskService
+    def tenant_root
+      File.join(root, Tenant.subfolder_path)
+    end
+
+    def tenant_root_for(...)
+      File.join(root, Tenant.subfolder_path_for(...))
+    end
+
     def path_for(key)
       if Tenant.default?
         super
       else
-        super.sub(root, File.join(root, Tenant.subfolder_path))
+        super.sub(root, tenant_root)
       end
     end
   end