fix(emqx_os_mon): call load_ctl (lc) to get accurate memory stats

Prior to this commit, the RAM usage ratio reported by memsup is
used to trigger EMQX alarm.
This ratio counted system cache as 'used', so quite often
a false alarm is triggered.
With lc 0.3.0, the usage is caculated without cached RAM

apps/emqx/src/emqx_os_mon.erl

@@ -77,17 +77,11 @@ set_procmem_high_watermark(Float) ->
     memsup:set_procmem_high_watermark(Float).
 
 current_sysmem_percent() ->
-    case erlang:whereis(memsup) of
-        undefined ->
+    case load_ctl:get_memory_usage() of
+        0 ->
             undefined;
-        _Pid ->
-            {Total, Allocated, _Worst} = memsup:get_memory_data(),
-            case Total =/= 0 of
-                true ->
-                    erlang:floor((Allocated / Total) * 10000) / 100;
-                false ->
-                    undefined
-            end
+        Ratio ->
+            erlang:floor(Ratio * 10000) / 100
     end.
 
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