bpf: allow BPF programs access skb_shared_info->gso_segs field

This adds the ability to read gso_segs from a BPF program.

v3: Use BPF_REG_AX instead of BPF_REG_TMP for the temporary register,
    as suggested by Martin.

v2: refined Eddie Hao patch to address Alexei feedback.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Eddie Hao <eddieh@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/include/uapi/linux/bpf.h

@@ -2540,6 +2540,7 @@ struct __sk_buff {
 	__bpf_md_ptr(struct bpf_flow_keys *, flow_keys);
 	__u64 tstamp;
 	__u32 wire_len;
+	__u32 gso_segs;
 };
 
 struct bpf_tunnel_key {