crypto: api - Fix races in crypto_unregister_instance

There are multiple problems in crypto_unregister_instance:

1) The cra_refcnt BUG_ON check is racy and can cause crashes.
2) The cra_refcnt check shouldn't exist at all.
3) There is no reference on tmpl to protect the tmpl->free call.

This patch rewrites the function using crypto_remove_spawn which
now morphs into crypto_remove_instance.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>