Alex Goins reported that mutex_destroy() on RT will force a GPL only symbol
which won't link and therefore fail on a non-GPL kernel module.
This does not happen on !RT and is a regression on RT which we would like to
avoid.
I try here the easy thing and to not use rt_mutex_destroy() if
CONFIG_DEBUG_MUTEXES is not enabled. This will still break for the DEBUG
configs so instead of adding a wrapper around rt_mutex_destroy() (which we have
for rt_mutex_lock() for instance) I am simply dropping the GPL part from the
export.

Reported-by: Alex Goins <redacted>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/mutex_rt.h |    5 +++++
 kernel/locking/rtmutex.c |    3 +--
 2 files changed, 6 insertions(+), 2 deletions(-)

--- a/include/linux/mutex_rt.h
+++ b/include/linux/mutex_rt.h

@@ -43,7 +43,12 @@ extern void __lockfunc _mutex_unlock(str
 #define mutex_lock_killable(l)		_mutex_lock_killable(l)
 #define mutex_trylock(l)		_mutex_trylock(l)
 #define mutex_unlock(l)			_mutex_unlock(l)
+
+#ifdef CONFIG_DEBUG_MUTEXES
 #define mutex_destroy(l)		rt_mutex_destroy(&(l)->lock)
+#else
+static inline void mutex_destroy(struct mutex *lock) {}
+#endif
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 # define mutex_lock_nested(l, s)	_mutex_lock_nested(l, s)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c

@@ -2027,8 +2027,7 @@ void rt_mutex_destroy(struct rt_mutex *l
 	lock->magic = NULL;
 #endif
 }
-
-EXPORT_SYMBOL_GPL(rt_mutex_destroy);
+EXPORT_SYMBOL(rt_mutex_destroy);