fix: support RKNN DMA buffer mmap on StarryOS
Return exported mmap-capable fd from RKNPU MemCreate, support DMA/RKNPU buffer mmap backends, and preserve allocation lifetime through VMA mappings. This fixes RKNN Runtime mmap failures and allows minimal_rknn_test to complete rknn_run, outputs_get, outputs_release, and destroy successfully on Orange Pi 5 Plus.