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>