xfs/194: unmount forced v4 fs during cleanup

xfs/194: unmount forced v4 fs during cleanup

Unmount the V4 filesystem we forcibly created to run this test during
test cleanup so that the post-test wrapup checks won't try to remount
the filesystem with different MOUNT_OPTIONS (specifically, the ones
that get screened out by _force_xfsv4_mount_options) and fail.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>