crypto: cts - simplify error handling in crypto_cts_create()

Simplify the error handling in crypto_cts_create() by taking advantage
of crypto_grab_skcipher() now handling an ERR_PTR() name and by taking
advantage of crypto_drop_skcipher() now accepting (as a no-op) a spawn
that hasn't been grabbed yet.

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