This revert commit c4baad50 ("virtio-console: avoid DMA from stack")

hvc framework will never pass stack memory to the put_chars() function,
So the calling of kmemdup() is unnecessary, we can remove it.
Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Reviewed-by: Shile Zhang <shile.zhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211015024658.1353987-4-xianting.tian@linux.alibaba.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>