crypto: tcrypt - fix AEAD decryption speed test

The AEAD speed test pretended to support decryption, however that support
was broken as decryption requires a valid auth field which the test did
not provide.

Fix this by running the encryption path once with inout/output sgls
switched to calculate the auth field prior to performing decryption
speed tests.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>