context_tracking, rcu: Rename RCU_DYNTICKS_IDX into CT_RCU_WATCHING

The symbols relating to the CT_STATE part of context_tracking.state are now
all prefixed with CT_STATE.

The RCU dynticks counter part of that atomic variable still involves
symbols with different prefixes, align them all to be prefixed with
CT_RCU_WATCHING.

Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>