Use ktime_to_ns/ns_to_ktime. This makes it possible to work with older kernels
and the media_build compatibility system.

For the mainline kernel these functions are NOPs.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>