context_tracking: Make context_tracking_key __ro_after_init

context_tracking_key is only ever enabled in __init ct_cpu_tracker_user(),
so mark it as __ro_after_init.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20240313180106.2917308-3-vschneid@redhat.com