rcu: Split rcu_report_exp_cpu_mult() mask parameter and use for tracing

This commit renames the rcu_report_exp_cpu_mult() function from "mask"
to "mask_in" and introduced a "mask" local variable to better support
upcoming event-tracing additions.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>