downgrade the log level of a proc_mutex_unlock error message

when it occurs during restart; it isn't at all uncommon then
when SysV semaphores are used for the accept mutex

same logic already in place for two other mutex calls


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@179317 13f79535-47bb-0310-9956-ffa450edef68