parameters: fix TSAN issue with AtomicTransaction

This fixes TSAN issues on destruction of AtomicTransaction happening
during the unit tests.

The static _MutexHolder instance wraps a pthread_mutex_t in a C++ class,
creating a destructor ordering problem during process exit when worker
threads may still hold the lock. Replace with a raw pthread_mutex_t
initialized via pthread_once, eliminating the C++ destructor entirely.

src/lib/parameters/atomic_transaction.cpp

@@ -34,5 +34,21 @@
 #include "atomic_transaction.h"
 
 #ifdef __PX4_POSIX
-_MutexHolder  AtomicTransaction::_mutex_holder = _MutexHolder {};
+static pthread_mutex_t _param_mutex;
+static pthread_once_t _param_mutex_once = PTHREAD_ONCE_INIT;
+
+static void _init_param_mutex()
+{
+	pthread_mutexattr_t attr;
+	pthread_mutexattr_init(&attr);
+	pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+	pthread_mutex_init(&_param_mutex, &attr);
+	pthread_mutexattr_destroy(&attr);
+}
+
+pthread_mutex_t *AtomicTransaction::_get_mutex()
+{
+	pthread_once(&_param_mutex_once, _init_param_mutex);
+	return &_param_mutex;
+}
 #endif

src/lib/parameters/atomic_transaction.h

@@ -39,25 +39,6 @@
 
 #ifdef __PX4_POSIX
 #include <pthread.h>
-
-class _MutexHolder
-{
-public:
-	pthread_mutex_t _mutex;
-	pthread_mutexattr_t _mutex_attr;
-
-	_MutexHolder()
-	{
-		pthread_mutexattr_init(&_mutex_attr);
-		pthread_mutexattr_settype(&_mutex_attr, PTHREAD_MUTEX_RECURSIVE);
-		pthread_mutex_init(&_mutex, &_mutex_attr);
-	}
-
-	~_MutexHolder()
-	{
-		pthread_mutex_destroy(&_mutex);
-	}
-};
 #endif
 
 
@@ -69,7 +50,7 @@ private:
 #endif
 
 #ifdef __PX4_POSIX
-	static _MutexHolder _mutex_holder;
+	static pthread_mutex_t *_get_mutex();
 #endif
 
 public:
@@ -89,7 +70,7 @@ public:
 		_irq_state = px4_enter_critical_section();
 #endif
 #ifdef __PX4_POSIX
-		pthread_mutex_lock(&_mutex_holder._mutex);
+		pthread_mutex_lock(_get_mutex());
 #endif
 	}
 
@@ -99,7 +80,7 @@ public:
 		px4_leave_critical_section(_irq_state);
 #endif
 #ifdef __PX4_POSIX
-		pthread_mutex_unlock(&_mutex_holder._mutex);
+		pthread_mutex_unlock(_get_mutex());
 #endif
 	}
 };