timers: Optimize get_timer_[this_]cpu_base()

If a timer is deferrable and NO_HZ_COMMON is enabled, get_timer_cpu_base()
and get_timer_this_cpu_base() invoke per_cpu_ptr() and this_cpu_ptr()
twice.

While this seems to be cheap, get_timer_cpu_base() can be called in a loop
in lock_timer_base().

Optimize the functions by updating the base index for deferrable timers and
retrieving the actual base pointer once.

In both cases the resulting assembly code of those helpers becomes smaller,
which results in a ~30% execution time reduction for a lock_timer_base()
micro bench mark.

Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20241231150115.1978342-1-quic_zhonhan@quicinc.com