cpuidle, nospec: Make mds_idle_clear_cpu_buffers() noinstr clean

objtool found that the mds_idle_clear_cpu_buffers() method got
uninlined by the compiler where it called out into instrumentation:

  vmlinux.o: warning: objtool: mwait_idle+0x47: call to mds_idle_clear_cpu_buffers() leaves .noinstr.text section
  vmlinux.o: warning: objtool: acpi_processor_ffh_cstate_enter+0xa2: call to mds_idle_clear_cpu_buffers() leaves .noinstr.text section
  vmlinux.o: warning: objtool: intel_idle+0x91: call to mds_idle_clear_cpu_buffers() leaves .noinstr.text section
  vmlinux.o: warning: objtool: intel_idle_s2idle+0x8c: call to mds_idle_clear_cpu_buffers() leaves .noinstr.text section
  vmlinux.o: warning: objtool: intel_idle_irq+0xaa: call to mds_idle_clear_cpu_buffers() leaves .noinstr.text section

Solve this by marking mds_idle_clear_cpu_buffers() as __always_inline.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230112195541.233779815@infradead.org

arch/x86/include/asm/nospec-branch.h

@@ -564,7 +564,7 @@ static __always_inline void mds_user_clear_cpu_buffers(void)
  *
  * Clear CPU buffers if the corresponding static key is enabled
  */
-static inline void mds_idle_clear_cpu_buffers(void)
+static __always_inline void mds_idle_clear_cpu_buffers(void)
 {
 	if (static_branch_likely(&mds_idle_clear))
 		mds_clear_cpu_buffers();