crypto: inside-secure - fix incorrect skcipher output IV

This patch fixes corruption issues with the skcipher output IV
witnessed on x86+EIP197-FPGA (devboard). The original fix, commit
57660b11 ("crypto: inside-secure - implement IV retrieval"),
attempted to write out the result IV through the context record.
However, this is not a reliable mechanism as there is no way of
knowing the hardware context update actually arrived in memory, so
it is possible to read the old contents instead of the updated IV.
(and indeed, this failed for the x86/FPGA case)

The alternative approach used here recognises the fact that the
result IV for CBC is actually the last cipher block, which is the last
input block in case of decryption and the last output block in case
of encryption. So the result IV is taken from the input data buffer
respectively the output data buffer instead, which *is* reliable.

Signed-off-by: Pascal van Leeuwen <pvanleeuwen@verimatrix.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>