mod_slotmem_shm: follow up to r1702450.

Rename ap_slotmem_instance_t's field 'name' to 'fname' since it really stores
the file path/name of the file-based SHM.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1702473 13f79535-47bb-0310-9956-ffa450edef68