crypto: af_alg - Drop support for off-CPU cryptography

crypto: af_alg - Drop support for off-CPU cryptography

AF_ALG is deprecated and exposed to unprivileged userspace.  Only
use the least buggy algorithm implementations: the pure software ones.

This removes one of the main advantages of AF_ALG, which is the
ability to use it with off-CPU accelerators.  However, using off-CPU
accelerators has huge overheads, both in performance and attack surface.
I have yet to see real-world, performance-critical workloads where using
an accelerator via AF_ALG is actually a win over doing cryptography in
userspace.

If using an off-CPU accelerator really does turn out to be a win, a new
API should be developed that is actually a good fit for it.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>