V4L/DVB (5568): VIDIOCGMBUF handling in video_ioctl2()

Correct handling of VIDIOCGMBUF in video_ioctl2().

Signed-off-by: Sam Revitch <samr7@cs.washington.edu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>