inode is never used on video_ioctl2. Remove it and rename the function to
__video_ioctl2. This allows its usage directly as a callback at
fops.unlocked_ioctl.

Since we still need a callback with inode to be used with fops.ioctl,
this patch adds video_ioctl2() that is just a call to __video_ioctl2().

Also, this patch adds some comments about video_ioctl2 and __video_ioctl2
usage at v4l2-ioctl.h.
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>