tick/nohz: Remove duplicate between tick_nohz_switch_to_nohz() and tick_setup_sched_timer()

The ts->sched_timer initialization work of tick_nohz_switch_to_nohz()
is almost the same as that of tick_setup_sched_timer(), so adjust the
latter to get it reused by tick_nohz_switch_to_nohz().

This also makes the low resolution mode sched_timer benefit from the tick
skew boot option.

Signed-off-by: Peng Liu <liupeng17@lenovo.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240225225508.11587-2-frederic@kernel.org