bpf: Fix issue in verifying allow_ptr_leaks

After we converted the capabilities of our networking-bpf program from
cap_sys_admin to cap_net_admin+cap_bpf, our networking-bpf program
failed to start. Because it failed the bpf verifier, and the error log
is "R3 pointer comparison prohibited".

A simple reproducer as follows,

SEC("cls-ingress")
int ingress(struct __sk_buff *skb)
{
	struct iphdr *iph = (void *)(long)skb->data + sizeof(struct ethhdr);

	if ((long)(iph + 1) > (long)skb->data_end)
		return TC_ACT_STOLEN;
	return TC_ACT_OK;
}

Per discussion with Yonghong and Alexei [1], comparison of two packet
pointers is not a pointer leak. This patch fixes it.

Our local kernel is 6.1.y and we expect this fix to be backported to
6.1.y, so stable is CCed.

[1]. https://lore.kernel.org/bpf/CAADnVQ+Nmspr7Si+pxWn8zkE7hX-7s93ugwC+94aXSy4uQ9vBg@mail.gmail.com/

Suggested-by: Yonghong Song <[email protected]>
Suggested-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Yafang Shao <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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