In preparation for adding inauthentic input fuzz tests, which don't
require that a generic implementation of the algorithm be available,
refactor test_aead_vs_generic_impl() so that instead there's a
higher-level function test_aead_extra() which initializes a struct
aead_extra_tests_ctx and then calls test_aead_vs_generic_impl() with a
pointer to that struct.

As a bonus, this reduces stack usage.

Also switch from crypto_aead_alg(tfm)->maxauthsize to
crypto_aead_maxauthsize(), now that the latter is available in
<crypto/aead.h>.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>