iob_reserve
If we apply `iob_reserve` on an IOB with `io_offset != 0`, the `head->io_pktlen` and `iob->io_len` will become wrong value, because we only need to trim `offset - iob->io_offset`. Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
Signed-off-by: chao an <anchao@xiaomi.com>