bcachefs: Fix rcu_pending for PREEMPT_RT

PREEMPT_RT redefines how standard spinlocks work, so local_irq_save() +
spin_lock() is no longer equivalent to spin_lock_irqsave(). Fortunately,
we don't strictly need to do it that way.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>