crypto: algapi - reject NULL crypto_spawn::inst

It took me a while to notice the bug where the adiantum template left
crypto_spawn::inst == NULL, because this only caused problems in certain
cases where algorithms are dynamically loaded/unloaded.

More improvements are needed, but for now make crypto_init_spawn()
reject this case and WARN(), so this type of bug will be noticed
immediately in the future.

Note: I checked all callers and the adiantum template was the only place
that had this wrong.  So this WARN shouldn't trigger anymore.

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