b43: fix debugfs crash

This patch fixes a crash that happens because b43's
debugfs code expects file->f_op to be a pointer to
its own b43_debugfs_fops struct. This is no longer
the case since commit 9fd4dcec
("debugfs: prevent access to possibly dead file_operations at file open")
Reviewed-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Cc: stable <stable@vger.kernel.org> # 4.7+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/wireless/broadcom/b43/debugfs.c

@@ -524,7 +524,8 @@ static ssize_t b43_debugfs_read(struct file *file, char __user *userbuf,
 		goto out_unlock;
 	}
 
-	dfops = container_of(file->f_op, struct b43_debugfs_fops, fops);
+	dfops = container_of(debugfs_real_fops(file),
+			     struct b43_debugfs_fops, fops);
 	if (!dfops->read) {
 		err = -ENOSYS;
 		goto out_unlock;
@@ -585,7 +586,8 @@ static ssize_t b43_debugfs_write(struct file *file,
 		goto out_unlock;
 	}
 
-	dfops = container_of(file->f_op, struct b43_debugfs_fops, fops);
+	dfops = container_of(debugfs_real_fops(file),
+			     struct b43_debugfs_fops, fops);
 	if (!dfops->write) {
 		err = -ENOSYS;
 		goto out_unlock;