selftests/bpf: Add skb_pkt_end test

Add a test that currently makes LLVM generate assembly code:

$ llvm-objdump -S skb_pkt_end.o
0000000000000000 <main_prog>:
; 	if (skb_shorter(skb, ETH_IPV4_TCP_SIZE))
       0:	61 12 50 00 00 00 00 00	r2 = *(u32 *)(r1 + 80)
       1:	61 14 4c 00 00 00 00 00	r4 = *(u32 *)(r1 + 76)
       2:	bf 43 00 00 00 00 00 00	r3 = r4
       3:	07 03 00 00 36 00 00 00	r3 += 54
       4:	b7 01 00 00 00 00 00 00	r1 = 0
       5:	2d 23 02 00 00 00 00 00	if r3 > r2 goto +2 <LBB0_2>
       6:	07 04 00 00 0e 00 00 00	r4 += 14
; 	if (skb_shorter(skb, ETH_IPV4_TCP_SIZE))
       7:	bf 41 00 00 00 00 00 00	r1 = r4
0000000000000040 <LBB0_2>:
       8:	b4 00 00 00 ff ff ff ff	w0 = -1
; 	if (!(ip = get_iphdr(skb)))
       9:	2d 23 05 00 00 00 00 00	if r3 > r2 goto +5 <LBB0_6>
; 	proto = ip->protocol;
      10:	71 12 09 00 00 00 00 00	r2 = *(u8 *)(r1 + 9)
; 	if (proto != IPPROTO_TCP)
      11:	56 02 03 00 06 00 00 00	if w2 != 6 goto +3 <LBB0_6>
; 	if (tcp->dest != 0)
      12:	69 12 16 00 00 00 00 00	r2 = *(u16 *)(r1 + 22)
      13:	56 02 01 00 00 00 00 00	if w2 != 0 goto +1 <LBB0_6>
; 	return tcp->urg_ptr;
      14:	69 10 26 00 00 00 00 00	r0 = *(u16 *)(r1 + 38)
0000000000000078 <LBB0_6>:
; }
      15:	95 00 00 00 00 00 00 00	exit

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201111031213.25109-3-alexei.starovoitov@gmail.com