dpif-netdev: Implement OVS_ACTION_ATTR_SAMPLE action.

dpif-netdev: Implement OVS_ACTION_ATTR_SAMPLE action.

OVS_ACTION_ATTR_SAMPLE has never been implemented in dpif-netdev.  This
commit implements it and adds a cast to enum ovs_action_type in the switch
statement that checks the action type, so that GCC complains if we forget
to add a case for a new action type.

I had to assign the return value of nl_attr_type() to a temporary variable,
because "switch ((enum ovs_action_type) nl_attr_type(a))" provoked a GCC
warning that I've never seen before:

../lib/dpif-netdev.c:1260: warning: cast from function call of type 'int'
     to non-matching type 'enum ovs_action_type'