aesni: Avoid modifying session keys in hmac_update()

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.

This is a direct commit to stable/12 as the bug is not present in later
branches.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D30001