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drivers, net, ethernet: convert mtk_eth.dma_refcnt from atomic_t to refcount_t
atomic_t variables are currently used to implement reference counters with the following properties: - counter is initialized to 1 using atomic_set() - a resource is freed upon counter reaching zero - once counter reaches zero, its further increments aren't allowed - counter schema uses basic atomic operations (set, inc, inc_not_zero, dec_and_test, etc.) Such atomic variables should be converted to a newly provided refcount_t type and API that prevents accidental counter overflows and underflows. This is important since overflows and underflows can lead to use-after-free situation and be exploitable. The variable mtk_eth.dma_refcnt is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by:Kees Cook <keescook@chromium.org> Reviewed-by: