datapath: Fix namespace refcount leak on failed init.

If a datapath fails to initialze fully (likely due to out-of-memory)
then it's possible that we can take a reference to a network
namespace but never release it.  This fixes the problem by releasing
any resources in the event of an error.

Found by code inspection, it's likely to be extremely rare in practice.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>