crypto: stm32 - Try to fix hash padding

crypto: stm32 - Try to fix hash padding

gcc warns that the length for the extra unaligned data in the hash
function may be used unaligned. In theory this could happen if
we pass a zero-length sg_list, or if sg_is_last() was never true:

In file included from drivers/crypto/stm32/stm32-hash.c:23:
drivers/crypto/stm32/stm32-hash.c: In function 'stm32_hash_one_request':
include/uapi/linux/kernel.h:12:49: error: 'ncp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
 #define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))

Neither of these can happen in practice, so the warning is harmless.

However while trying to suppress the warning, I noticed multiple
problems with that code:

- On big-endian kernels, we byte-swap the data like we do for
  register accesses, however this is a data stream and almost
  certainly needs to use a single writesl() instead of series
  of writel() to give the correct hash.

- If the length is not a multiple of four bytes, we skip the
  last word entirely, since we write the truncated length
  using stm32_hash_set_nblw().

- If we change the code to round the length up rather than
  down, the last bytes contain stale data, so it needs some
  form of padding.

This tries to address all four problems, by correctly
initializing the length to zero, using endian-safe copy
functions, adding zero-padding and passing the padded length.

I have done no testing on this patch, so please review
carefully and if possible test with an unaligned length
and big-endian kernel builds.

Fixes: 8a1012d3f2ab ("crypto: stm32 - Support for STM32 HASH module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>