audit: Fix overflow in file backend

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)

Change-Id: Ie364aae0749b3658ab11a354844878d10c6970ab
Reviewed-on: http://gerrit.openafs.org/9448
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>