mod_dumpio: do nothing below log level TRACE7.

For instance, depending on EnableMMAP/Sendfile configuration, don't split
file brigades to 8K heap buckets upon reading.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1818802 13f79535-47bb-0310-9956-ffa450edef68