x86/fpu: Make kernel FPU protection RT friendly

Non RT kernels need to protect FPU against preemption and bottom half
processing. This is achieved by disabling bottom halfs via
local_bh_disable() which implictly disables preemption.

On RT kernels this protection mechanism is not sufficient because
local_bh_disable() does not disable preemption. It serializes bottom half
related processing via a CPU local lock.

As bottom halfs are running always in thread context on RT kernels
disabling preemption is the proper choice as it implicitly prevents bottom
half processing.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201027101349.588965083@linutronix.de