crypto: adiantum - use crypto_grab_{cipher,shash} and simplify error paths

Make the adiantum template use the new functions crypto_grab_cipher()
and crypto_grab_shash() to initialize its cipher and shash spawns.

This is needed to make all spawns be initialized in a consistent way.

Also simplify the error handling by taking advantage of crypto_drop_*()
now accepting (as a no-op) spawns that haven't been initialized yet, and
by taking advantage of crypto_grab_*() now handling ERR_PTR() names.

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