config: fix selinux context handling

config: fix selinux context handling

With the new config stuff we lost the selinux options being set for systems with
selinux turned on.  We want the selinux context set all the time, wether we
provide a MOUNT_OPTIONS value or not, so take this logic out of _mount_opts()
and just put it in the body of common/config

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>