xfstests: change directory to / before _cleanup_testdir in test 135

[Boris Ranto <branto@xxxxxxxxxx>]

Nfs tries to umount $testdir in _cleanup_testdir function. The test 135 calls 
the function from directory $SCRATCH_MNT that is equal to $testdir (at least 
for nfs). The umount will therefore fail causing the test to fail due to the 
output mismatch.

This simple patch fixes the issue for me.

Signed-off-by: Boris Ranto <branto@xxxxxxxxxx>

diff -urpN a/xfstests/135 b/xfstests/135
--- a/xfstests/135      2011-03-07 14:54:15.855172101 +0100
+++ b/xfstests/135      2011-03-07 14:54:29.895048375 +0100
@@ -34,6 +34,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 1
 
 _cleanup()
 {
+    cd /
     _cleanup_testdir
 }