libsmack: close smackfs_mnt_dirfd in the library destructor

Close smackfs_mnt_dirfd in the library destructor. Although kernel
would wipe it anyway it is a good practice to clean up all the
reserved resources.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
libsmack: lazy initialization for SmackFS mount point

Mount SmackFS only when it is first needed. This enables init
daemons to directly use libsmack.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>