posix-timers: Initialize cache early and move pointer into __timer_data

Move posix_timers_cache initialization to posixtimer_init(). At that point
the memory subsystem is already up and running.

Also move the cache pointer to the __timer_data variable to avoid
potential false sharing, since it never was marked as __ro_after_init.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250402133114.253901-1-edumazet@google.com