Now that crypto_init_spawn() is only called by crypto_grab_spawn(),
simplify things by moving its functionality into crypto_grab_spawn().

In the process of doing this, also be more consistent about when the
spawn and instance are updated, and remove the crypto_spawn::dropref
flag since now it's always set.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>