crypto: ctr - simplify error handling in crypto_rfc3686_create()

Simplify the error handling in crypto_rfc3686_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>