datapath: Add support for OVS_FLOW_ATTR_PROBE.

This new flag is useful for suppressing error logging while probing
for datapath features using flow commands.  For backwards
compatibility reasons the commands are executed normally, but error
logging is suppressed.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
dpif: Use OVS_FLOW_ATTR_PROBE.

Use the new OVS_FLOW_ATTR_PROBE flag when probing for datapath feature
support.  Suppress also dpif error logging when requested, as probe
failures are already logged at ofproto-dpif.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>