drm/vc4: Supply the overflow slot size in BPOS, not the whole bin BO size

In the Linux kernel, the following vulnerability has been resolved: drm/vc4: Supply the overflow slot size in BPOS, not the whole bin BO size vc4_overflow_mem_work() points BPOA at a 512KB slot inside the 16MB binner BO, but writes the size of the whole BO to BPOS. On every binner out-of-memory event the PTB is therefore authorized to write tile lists across all the other slots (which may hold the tile state, tile alloc and overflow memory of in-flight jobs) and, for any slot but the first, past the end of the binner BO into unrelated CMA memory. Since CMA pages are recycled into page cache and user allocations, this is arbitrary memory corruption by GPU DMA. In practice it shows up as GPU hangs with corrupted control list pointers, userspace heap corruption, a GPU that stays permanently wedged after the first hang, and occasional full system crashes, whenever a job overflows the initial binner slot. The bug dates back to the conversion from a dedicated overflow BO (where writing the full BO size was correct) to the slotted binner BO.