debugfs: file: Remove unnecessary cast in kfree()

Remove unnecassary casts in the argument to kfree.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20200709054033.30148-1-vulab@iscas.ac.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>