drivers/staging/exfat: Replace binary semaphores for mutexes

At a slight footprint cost (24 vs 32 bytes), mutexes are more optimal
than semaphores; it's also a nicer interface for mutual exclusion,
which is why they are encouraged over binary semaphores, when possible.

For both v_sem and z_sem, their semantics imply traditional lock
ownership; that is, the lock owner is the same for both lock/unlock
operations. Therefore it is safe to convert.
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Acked-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191030144916.10802-1-dave@stgolabs.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/exfat/exfat.h

@@ -610,7 +610,7 @@ struct fs_info_t {
 	u32 dev_ejected;	/* block device operation error flag */
 
 	struct fs_func *fs_func;
-	struct semaphore v_sem;
+	struct mutex v_mutex;
 
 	/* FAT cache */
 	struct buf_cache_t FAT_cache_array[FAT_CACHE_SIZE];