ossl: Keep mutable AES-GCM state on the stack

ossl(4)'s AES-GCM implementation keeps mutable state in the session
structure, together with the key schedule. This was done for
convenience, as both are initialized together. However, some OCF
consumers, particularly ZFS, assume that requests may be dispatched to
the same session in parallel. Without serialization, this results in
incorrect output.

Fix the problem by explicitly copying per-session state onto the stack
at the beginning of each operation.

PR: 275306
Reviewed by: jhb
Fixes: 9a3444d91c70 ("ossl: Add a VAES-based AES-GCM implementation for amd64")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42783

(cherry picked from commit 5c0dac0b7a012f326edab06ad85aee5ad68ff120)