mmap: remove dup in mnl_mmap_recv() / fix tx offset mamange

sorry for two issue.
mnl_mmap_recv() can implement by wrapping mnl_mmap_recvhdr()
mmap: manage rx status by pointer

change struct member last_status to rx_current
mmap: (still working, halfway) imitate struct to kernel, origin/nl-mmap

as possible as I can.