EXP refscale: Make scale_type=bh safe for PREEMPT_RT kernels

Kernels built with CONFIG_PREEMPT_RT=y do not permit using
local_bh_disable() while preemption is disabled, so skip disabling
preemption on such kernels.  Longer term, there might be a fix in -rt,
so just keep this for local use for the moment.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Closes: https://lore.kernel.org/all/20251111153803.5_08_ro2@linutronix.de/
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>