[media] v4l2-dev.h: add comment not to use V4L2_FL_LOCK_ALL_FOPS in new drivers

This flag is for legacy drivers only and will go away in the future.

A note regarding commit 5126f259
(v4l2-dev: add flag to have the core lock all file operations):

That commit message suggests that by not taking the core lock for fops
other than unlocked_ioctl all problems relating to AB-BA locking and
mm->mmap_sem are solved. This is not the case.

More work needs to be done by moving the core lock further down into
video_ioctl2. It should only be taken after the copy_from/to_user calls
are done.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>