dpif-linux: fix the size of n_masks

dpif-linux: fix the size of n_masks

The command ovs-dpctl can wrongly output the masks even if the
datapath does not implement mega flows. In this case the output
will be similar to the following:

system@ovs-system:
	lookups: hit:14 missed:41 lost:0
	flows: 0
	masks: hit:18446744073709551615 total:4294967295
		hit/pkt:335395346794719104.00
	port 0: ovs-system (internal)
	port 1: gre_system (gre: df_default=false, ttl=0)
	port 2: ots-br0 (internal)
	port 3: int0 (internal)
	port 4: vnet0
	port 5: vnet1

The problem depends on the fact that n_masks stats is stored as a
uint32 in the struct ovs_dp_megaflow_stats and as a uint64 in the
struct dpif_dp_stats. UINT32_MAX instead of UINT64_MAX should be
used to detect if the datapath supports megaflows or not.

Signed-off-by: Francesco Fusco <ffusco@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>