openvswitch: Change CT_ATTR_FLAGS to CT_ATTR_COMMIT

Previously, the CT_ATTR_FLAGS attribute, when nested under the
OVS_ACTION_ATTR_CT, encoded a 32-bit bitmask of flags that modify the
semantics of the ct action. It's more extensible to just represent each
flag as a nested attribute, and this requires no additional error
checking to reject flags that aren't currently supported.

Suggested-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>