ima: fix wrong signed policy requirement when not appraising

Kernel booted just with ima_policy=tcb (not with
ima_policy=appraise_tcb) shouldn't require signed policy.

Regression found with LTP test ima_policy.sh.

Fixes: c52657d9

 ("ima: refactor ima_init_policy()")
Cc: stable@vger.kernel.org  (linux-5.0)
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>