crypto: skcipher - remove unnecessary setting of walk->nbytes

Setting 'walk->nbytes = walk->total' in skcipher_walk_first() doesn't
make sense because actually walk->nbytes needs to be set to the length
of the first step in the walk, which may be less than walk->total.  This
is done by skcipher_walk_next() which is called immediately afterwards.
Also walk->nbytes was already set to 0 in skcipher_walk_skcipher(),
which is a better default value in case it's forgotten to be set later.

Therefore, remove the unnecessary assignment to walk->nbytes.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>