timers/migration: Mark updates to tmigr_event ->field

The tmigr_event structure's ->ignore field are sometimes accessed
locklessly, but the __tmigr_cpu_activate(), tmigr_cpu_new_timer(),
and __tmigr_cpu_deactivate() functions do not mark accesses to this field.

Therefore, use READ_ONCE() for the tmigr_cpu_new_timer() function's
lockless load and WRITE_ONCE() for the __tmigr_cpu_activate() and
__tmigr_cpu_deactivate() functions' stores.

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@kernel.org>