rust: replace lsm context+len with lsm_context

This brings the Rust SecurityCtx abstraction [1] up to date with the new
API where context+len is replaced with an lsm_context [2] struct.

Link: https://lore.kernel.org/r/20240915-alice-file-v10-5-88484f7a3dcf@google.com [1]
Link: https://lore.kernel.org/r/20241023212158.18718-3-casey@schaufler-ca.com

 [2]
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Closes: https://lore.kernel.org/r/CA+G9fYv_Y2tzs+uYhMGtfUK9dSYV2mFr6WyKEzJazDsdk9o5zw@mail.gmail.com


Signed-off-by: Alice Ryhl <aliceryhl@google.com>
[PM: subj line tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>