bpf: remove redundant null pointer check before consume_skb

consume_skb has taken the null pointer into account. hence it is safe
to remove the redundant null pointer check before consume_skb.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>