crypto: algif_aead - copy AAD from src to dst

Use the NULL cipher to copy the AAD and PT/CT from the TX SGL
to the RX SGL. This allows an in-place crypto operation on the
RX SGL for encryption, because the TX data is always smaller or
equal to the RX data (the RX data will hold the tag).

For decryption, a per-request TX SGL is created which will only hold
the tag value. As the RX SGL will have no space for the tag value and
an in-place operation will not write the tag buffer, the TX SGL with the
tag value is chained to the RX SGL. This now allows an in-place
crypto operation.

For example:

* without the patch:
kcapi -x 2 -e -c "gcm(aes)" -p 89154d0d4129d322e4487bafaa4f6b46 -k c0ece3e63198af382b5603331cc23fa8 -i 7e489b83622e7228314d878d -a afcd7202d621e06ca53b70c2bdff7fb2 -l 16 -u -s
00000000000000000000000000000000f4a3eacfbdadd3b1a17117b1d67ffc1f1e21efbbc6d83724a8c296e3bb8cda0c

* with the patch:
kcapi -x 2 -e -c "gcm(aes)" -p 89154d0d4129d322e4487bafaa4f6b46 -k c0ece3e63198af382b5603331cc23fa8 -i 7e489...