security/smack: remove unused varible 'rc'

This varible isn't used and can be removed to avoid a gcc warning:
security/smack/smack_lsm.c:3873:6: warning: variable ‘rc’ set but not
used [-Wunused-but-set-variable]

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>