bpf: fix return in bpf_skb_adjust_net

The bpf_skb_adjust_net() ignores the return value of bpf_skb_net_shrink/grow,
and always return 0, fix it by return 'ret'.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>