bug-867253.t: do a clean umount at the end

On some machines, not doing a clean umount of NFS mount results
in a hang, because sometimes the next script's cleanup() kills
NFS server and the client mountpoint hangs forever.

Change-Id: I64798202180c920650272cc19bcbe07d44377ff6
BUG: 867253
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4421
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>