If there is no active IP session (interface up & running) then
release the data channel.

Use nr_sessions variable to track current active IP sessions.
If the count drops to 0, then send pipe close ctrl message to
release the data channel.
Signed-off-by: NM Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: NSergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>