audit: Fix overflow in file backend

If the filename passed to open_file was larger than MAXPATHLEN-5,
then we'd overflow the oldName buffer when creating the backup
filename. Fix the overflow by using a malloc'd buffer instead.

Caught by coverity (#985767)

Reviewed-on: http://gerrit.openafs.org/9448
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit b0b3def56c15161df28059e270f0360c31241217)

Change-Id: I3993de8e4372c30d35e6e675042511f85ba9d014
Reviewed-on: http://gerrit.openafs.org/11062
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>