crypto: nx - respect sg limit bounds when building sg lists for SHA

Commit 00085111 changed sha256/512 update functions to
pass more data to nx_build_sg_list(), which ends with
sg list overflows and usually with update functions failing
for data larger than max_sg_len * NX_PAGE_SIZE.

This happens because:
- both "total" and "to_process" are updated, which leads to
  "to_process" getting overflowed for some data lengths
  For example:
    In first iteration "total" is 50, and let's assume "to_process"
    is 30 due to sg limits. At the end of first iteration "total" is
    set to 20. At start of 2nd iteration "to_process" overflows on:
      to_process = total - to_process;
- "in_sg" is not reset to nx_ctx->in_sg after each iteration
- nx_build_sg_list() is hitting overflow because the amount of data
  passed to it would require more than sgmax elements
- as consequence of previous item, data stored in overflowed sg list
  may no longer be aligned to SHA*_BLOCK_SIZE

This patch changes sha256/512 update functions so that "to_process"
respects sg limits and never tries to pass more data to
nx_build_sg_list() to avoid overflows. "to_process" is calculated
as minimum of "total" and sg limits at start of every iteration.

Fixes: 00085111 ("crypto: nx - Fix SHA concurrence issue and sg
		      limit bounds")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Cc: stable@vger.kernel.org
Cc: Leonidas Da Silva Barbosa <leosilva@linux.vnet.ibm.com>
Cc: Marcelo Henrique Cerri <mhcerri@linux.vnet.ibm.com>
Cc: Fionnuala Gunter <fin@linux.vnet.ibm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>