media: v4l2-core: add v4l2_debugfs_root()
This new function returns the dentry of the top-level debugfs "v4l2" directory. If it does not exist yet, then it is created first.
Signed-off-by: Hans Verkuil hverkuil-cisco@xs4all.nl Signed-off-by: Mauro Carvalho Chehab mchehab+huawei@kernel.org