MemoryIdler: use mallctl directly for tcache.flush
MemoryIdler: use mallctl directly for tcache.flush

Summary:
tcache.flush may fail if tcache is disabled.  Avoid using mallctlCall
which throws on error.

Reviewed By: davidtgoldblatt

Differential Revision: D6115419

fbshipit-source-id: 39411c80af08dc7c855efd43297809b749f935bf

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