crypto: testmgr - fix out of bound read in __test_aead()

__test_aead() reads MAX_IVLEN bytes from template[i].iv, but the
actual length of the initialisation vector can be shorter.
The length of the IV is already calculated earlier in the
function. Let's just reuses that. Also the IV length is currently
calculated several time for no reason. Let's fix that too.
This fix an out-of-bound error detected by KASan.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>