futex: Use RCU_INIT_POINTER() in futex_mm_init().

futex: Use RCU_INIT_POINTER() in futex_mm_init().

There is no need for an explicit NULL pointer initialisation plus a
comment why it is okay. RCU_INIT_POINTER() can be used for NULL
initialisations and it is documented.

This has been build tested with gcc version 9.3.0 (Debian 9.3.0-22) on a
x86-64 defconfig.

Fixes: 094ac8cff7858 ("futex: Relax the rcu_assign_pointer() assignment of mm->futex_phash in futex_mm_init()")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250517151455.1065363-4-bigeasy@linutronix.de