The fact that an error in the skcipher walker API are indicated
not only by a non-zero return value, but also by the fact that
walk->nbytes is zero, causes the layout of the skcipher walker
loop to be sufficiently different from the usual layout, which
is not a problem in itself, but it is likely to cause reading
confusion and difficulty in code maintenance.

This patch rewrites skcipher walker loop, and separates the
last chunk cryption from the loop to avoid wrong calls to the
skcipher walker API. In addition to following the usual convention
of checking walk->nbytes, it also makes the loop execute logic
clearer and easier to understand.
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>