quick selinux support for tracefs

Here is just the quick fix for tracefs with selinux.
just add tracefs to the list of whitelisted filesystem
types in selinux_is_sblabel_mnt(), but the right fix would be to
generalize this logic as described in the last item on the todo list,
https://bitbucket.org/seandroid/wiki/wiki/ToDo



Change-Id: I2aa803ccffbcd2802a7287514da7648e6b364157
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>