Smack: prevent underflow in smk_set_cipso()

We have an upper bound on "maplevel" but forgot to check for negative
values.

Fixes: e114e473 ("Smack: Simplified Mandatory Access Control Kernel")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>