openvswitch: allocate nr_node_ids flow_stats instead of num_possible_nodes

Some architectures like POWER can have a NUMA node_possible_map that
contains sparse entries. This causes memory corruption with openvswitch
since it allocates flow_cache with a multiple of num_possible_nodes() and
assumes the node variable returned by for_each_node will index into
flow->stats[node].

Use nr_node_ids to allocate a maximal sparse array instead of
num_possible_nodes().

The crash was noticed after 3af229f2 was applied as it changed the
node_possible_map to match node_online_map on boot.
Fixes: 3af229f2

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>