locking/doc: Update references to kernel/mutex.c

Fix this docbook error:

  >> docproc: kernel/mutex.c: No such file or directory

by updating the stale references to kernel/mutex.c.

Reported-by: fengguang.wu@intel.com
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-34pikw1tlsskj65rrt5iusrq@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/mutex.h

@@ -131,7 +131,7 @@ static inline int mutex_is_locked(struct mutex *lock)
 }
 
 /*
- * See kernel/mutex.c for detailed documentation of these APIs.
+ * See kernel/locking/mutex.c for detailed documentation of these APIs.
  * Also see Documentation/mutex-design.txt.
  */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC