lsm,rust: reword "destroy" -> "release" in SecurityCtx

What happens inside the individual LSMs for a given LSM hook can vary
quite a bit, so it is best to use the terminology "release" instead of
"destroy" or "free".

Suggested-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
[PM: subj tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>