crypto: aead - move crypto_aead_maxauthsize() to <crypto/aead.h>

Move crypto_aead_maxauthsize() to <crypto/aead.h> so that it's available
to users of the API, not just AEAD implementations.

This will be used by the self-tests.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>