crypto: algif_aead - fix AEAD tag memory handling

For encryption, the AEAD ciphers require AAD || PT as input and generate
AAD || CT || Tag as output and vice versa for decryption. Prior to this
patch, the AF_ALG interface for AEAD ciphers requires the buffer to be
present as input for encryption. Similarly, the output buffer for
decryption required the presence of the tag buffer too. This implies
that the kernel reads / writes data buffers from/to kernel space
even though this operation is not required.

This patch changes the AF_ALG AEAD interface to be consistent with the
in-kernel AEAD cipher requirements.

Due to this handling, he changes are transparent to user space with one
exception: the return code of recv indicates the mount of output buffer.
That output buffer has a different size compared to before the patch
which implies that the return code of recv will also be different.
For example, a decryption operation uses 16 bytes AAD, 16 bytes CT and
16 bytes tag, the AF_ALG AEAD interface before showed a recv return
code of 48 (bytes) whereas after this patch, the return code is 32
since the tag is not returned any more.

Reported-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>