context_tracking: Rename context_tracking_active() to context_tracking_cpu_is_enabled()

We currently have a confusing couple of API naming with the existing
context_tracking_active() and context_tracking_is_enabled().

Lets keep the latter one, context_tracking_is_enabled(), for global
context tracking state check and use context_tracking_cpu_is_enabled()
for local state check.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>