core_output_filter: use the current input brigade's pool for the "more"

brigade when we defer network writes due to small data + keepalives.  This
prevents leaking resources in the "more" brigade until the end of the
connection.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97503 13f79535-47bb-0310-9956-ffa450edef68