x86/microcode: Remove microcode_mutex

microcode_mutex is only used by reload_store(). It has a comment saying
"to synchronize with each other". Other user of this mutex have been
removed in the commits

  181b6f40 ("x86/microcode: Rip out the OLD_INTERFACE").
  b6f86689 ("x86/microcode: Rip out the subsys interface gunk")

The sysfs interface does not need additional synchronisation vs itself
because it is provided as kernfs_ops::mutex which is acquired in
kernfs_fop_write_iter().

Remove the superfluous microcode_mutex.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230804075853.JF_n6GXC@linutronix.de

arch/x86/kernel/cpu/common.c

@@ -2276,8 +2276,7 @@ void store_cpu_caps(struct cpuinfo_x86 *curr_info)
  * @prev_info:	CPU capabilities stored before an update.
  *
  * The microcode loader calls this upon late microcode load to recheck features,
- * only when microcode has been updated. Caller holds microcode_mutex and CPU
- * hotplug lock.
+ * only when microcode has been updated. Caller holds and CPU hotplug lock.
  *
  * Return: None
  */

arch/x86/kernel/cpu/microcode/core.c

@@ -54,15 +54,12 @@ LIST_HEAD(microcode_cache);
  *
  * All non cpu-hotplug-callback call sites use:
  *
- * - microcode_mutex to synchronize with each other;
  * - cpus_read_lock/unlock() to synchronize with
  *   the cpu-hotplug-callback call sites.
  *
  * We guarantee that only a single cpu is being
  * updated at any particular moment of time.
  */
-static DEFINE_MUTEX(microcode_mutex);
-
 struct ucode_cpu_info		ucode_cpu_info[NR_CPUS];
 
 struct cpu_info_ctx {
@@ -488,10 +485,7 @@ static ssize_t reload_store(struct device *dev,
 	if (tmp_ret != UCODE_NEW)
 		goto put;
 
-	mutex_lock(&microcode_mutex);
 	ret = microcode_reload_late();
-	mutex_unlock(&microcode_mutex);
-
 put:
 	cpus_read_unlock();