Avoid net_kernel startup/shutdown races when peeking into ETS sys_dist table

Avoid errors like this:

    2013-08-08 14:55:26.301 [error] <0.7207.0> riak_sysmon_filter:get_node_map: error badarg @ [{ets,match_object,[sys_dist,'_'],[]},{ets,tab2list,1,[{file,"ets.erl"},{line,323}]},{riak_sysmon_filter,get_node_map,0,[{file,"src/riak_sysmon_filter.erl"},{line,360}]},{riak_sysmon_filter,handle_info,2,[{file,"src/riak_sysmon_filter.erl"},{line,249}]},{gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,607}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,227}]}]