smack: label /etc/mtab as "_" when '--with-smack-run-label' is enabled.

/etc/mtab should be labeled as "_", even though systemd has its own
smack label using '--with-smack-run-label' configuration. This is mainly
because all processes could read that file and the origin of this file
(i.e. /proc/mounts) is labeled as "_". This labels /etc/mtab as "_" when
'--with-smack-run-label' is enabled.