patch: rt: preempt_disable_rt for CONFIG_PREEMPT_RT

in some cases, spinlocks are used to avoid preemption. does not work
correctly, when CONFIG_PREEMPT_RT is enabled.

therefore two new macros are introduced:
preempt_disable_rt/preempt_disable_rt behave like their equivalents, if
CONFIG_PREEMPT_RT is enabled, and as noops otherwise.