Currently, crypto_spawn::inst is first used temporarily to pass the
instance to crypto_grab_spawn().  Then crypto_init_spawn() overwrites it
with crypto_spawn::next, which shares the same union.  Finally,
crypto_spawn::inst is set again when the instance is registered.

Make this less convoluted by just passing the instance as an argument to
crypto_grab_spawn() instead.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>