aesni: Avoid modifying session keys in hmac_update()

[markj]

Summary

Otherwise aesni_process() is not thread-safe for AES+SHA-HMAC
transforms, since hmac_update() updates the caller-supplied key directly
to create the derived key. Use a buffer on the stack to store a copy of
the key used for computing inner and outer digests.

Note that this applies to stable/12 only.