xstat: Ignore return value from xstat_*_Cleanup

We're about to exit anyway, so we don't care whether Cleanup
succeeds or not

Caught by clang-analyzer

Change-Id: I6350d60e6a5d29b8aaa272a54cf9ead47fdb5fad
Reviewed-on: http://gerrit.openafs.org/9186
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>