crypto: lrw - simplify error handling in create()

Simplify the error handling in the LRW template's ->create() function 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>