crypto: hash - Fix the pointer voodoo in unaligned ahash

Add documentation for the pointer voodoo that is happening in crypto/ahash.c
in ahash_op_unaligned(). This code is quite confusing, so add a beefy chunk
of documentation.

Moreover, make sure the mangled request is completely restored after finishing
this unaligned operation. This means restoring all of .result, .base.data
and .base.complete .

Also, remove the crypto_completion_t complete = ... line present in the
ahash_op_unaligned_done() function. This type actually declares a function
pointer, which is very confusing.

Finally, yet very important nonetheless, make sure the req->priv is free()'d
only after the original request is restored in ahash_op_unaligned_done().
The req->priv data must not be free()'d before that in ahash_op_unaligned_finish(),
since we would be accessing previously free()'d data in ahash_op_unaligned_done()
and cause corruption.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>