netfilter: xt_IDLETIMER: Use uevent instead of new netlink msg type

Send netlink message notifications using a uevent with
  subsystem=xt_idletimer
  INTERFACE=...
  STATE={active,inactive}
instead of needing a new netlink message type.
Revert the netlink.h change added by commit
  beb914e987cbbd368988d2b94a6661cb907c4d5a

Change-Id: I6881936bf754f3367ff3c8f6c1e9661cec0357d4
Signed-off-by: JP Abgrall <jpa@google.com>