datapath: Fix deadlock on STT device destroy.

datapath: Fix deadlock on STT device destroy.

STT unregisters nf-hook when there are no other STT devices
left in the namespace. On some kernel versions the nf-unreg API
take RTNL lock, but it is already taken in the tunnel device
destroy code path which results in deadlock. To fix the issue
I moved the unreg call into net-exit.

VMware-BZ: #1582410
Reported-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Joe Stringer <joe@ovn.org>