netmap_obj_offset returns 0 on error indistinguishable from valid offset 0: masks failures

Summary

netmap_obj_offset(:227-261): returns 0 to mean not in any cluster(:247) but 0 is also valid offset of object 0. Callers(netmap_ring_offset/netmap_buf_offset macros :254-261 netmap_mem_if_new :1065-1072) cannot detect error. If ring pointer not in its pool(UAF/wrong allocator) failure silently treated as offset 0 embedded into ring_ofs/nifp -> userspace dereferences wrong shared-memory location. Fix: return distinguished error or out-param int error.