datapath: Fully initialize datapath before local port.

datapath: Fully initialize datapath before local port.

It's possible to start receiving packets on a datapath as soon as
the internal device is created.  It's therefore important that the
datapath be fully initialized before this, which it currently isn't.
In particular, the fact that dp->stats_percpu is not yet set is
potentially fatal.  In addition, if allocation of the Netlink response
failed it would leak the percpu memory.  This fixes both problems.

Found by code inspection, in practice the datapath is probably always
done initializing before someone can send a packet on it.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>