x86/fpu: Simplify fpregs_[un]lock()

There is no point in disabling preemption and then disabling bottom
halfs.

Just disabling bottom halfs is sufficient as it implicitly disables
preemption on !RT kernels.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201027101349.455380473@linutronix.de

arch/x86/include/asm/fpu/api.h

@@ -29,17 +29,18 @@ extern void fpregs_mark_activate(void);
  * A context switch will (and softirq might) save CPU's FPU registers to
  * fpu->state and set TIF_NEED_FPU_LOAD leaving CPU's FPU registers in
  * a random state.
+ *
+ * local_bh_disable() protects against both preemption and soft interrupts
+ * on !RT kernels.
  */
 static inline void fpregs_lock(void)
 {
-	preempt_disable();
 	local_bh_disable();
 }
 
 static inline void fpregs_unlock(void)
 {
 	local_bh_enable();
-	preempt_enable();
 }
 
 #ifdef CONFIG_X86_DEBUG_FPU