smpboot: Mark idle_init() as __always_inlined to work around aggressive compiler un-inlining

While this function is a static inline, and is only used once in
local scope, certain Kconfig variations may cause it to be compiled
as a standalone function:

  89231bf0 <idle_init>:
  89231bf0:       83 05 60 d9 45 89 01    addl   $0x1,0x8945d960
  89231bf7:       55                      push   %ebp

Resulting in this build failure:

  WARNING: modpost: vmlinux.o(.text.unlikely+0x7fd5): Section mismatch in reference from the function idle_init() to the function .init.text:fork_idle()
  The function idle_init() references
  the function __init fork_idle().
  This is often because idle_init lacks a __init
  annotation or the annotation of fork_idle is wrong.
  ERROR: modpost: Section mismatches detected.

Certain USBSAN options x86-32 builds with CONFIG_CC_OPTIMIZE_FOR_SIZE=y
seem to be causing this.

So mark idle_init() as __always_inline to work around this compiler
bug/feature.
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/smpboot.c

@@ -47,7 +47,7 @@ void __init idle_thread_set_boot_cpu(void)
  *
  * Creates the thread if it does not exist.
  */
-static inline void idle_init(unsigned int cpu)
+static inline void __always_inline idle_init(unsigned int cpu)
 {
 	struct task_struct *tsk = per_cpu(idle_threads, cpu);