libmount: remove smackfs* option when SMACK not enabled

If there is no /sys/fs/smackfs then libmount removes smackfs*= mount
options when compiled --with-smack.

Note that we do the same for SELinux.

References: http://www.mail-archive.com/systemd-devel@lists.freedesktop.org/msg13740.html
Signed-off-by: Karel Zak <kzak@redhat.com>