crypto: algif_skcipher - Fix stream cipher chaining

Unlike algif_aead which is always issued in one go (thus limiting
the maximum size of the request), algif_skcipher has always allowed
unlimited input data by cutting them up as necessary and feeding
the fragments to the underlying algorithm one at a time.

However, because of deficiencies in the API, this has been broken
for most stream ciphers such as arc4 or chacha.  This is because
they have an internal state in addition to the IV that must be
preserved in order to continue processing.

Fix this by using the new skcipher state API.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>