Use memdup_user_nul() helper instead of open-coding to simplify the
code.
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/6baf3aa45d0b5e0fd016b508bac905ebf8443aac.1493779294.git.geliangtang@gmail.com