media: dvbdev: Initialize sbuf

[ Upstream commit 17d1316de0d7dc1bdc5d6e3ad4efd30a9bf1a381 ]

Because the size passed to copy_from_user() cannot be known beforehand,
it needs to be checked during runtime with check_object_size. That makes
gcc believe that the content of sbuf can be used before init.

Fix:
./include/linux/thread_info.h:215:17: warning: ‘sbuf’ may be used uninitialized [-Wmaybe-uninitialized]

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sasha Levin <sashal@kernel.org>