crypto: testmgr - limit IV copy length in aead tests

The working copy of IV is the same size as the transformation's IV.
It is not necessary to copy more than that from the template since
iv_len is usually less than MAX_IVLEN and the rest of the copied data
is garbage.

Signed-off-by: Cristian Stoica <cristian.stoica@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>