bpf: Extend the sk_lookup() helper to XDP hookpoint.

This patch proposes to extend the sk_lookup() BPF API to the
XDP hookpoint. The sk_lookup() helper supports a lookup
on incoming packet to find the corresponding socket that will
receive this packet. Current support for this BPF API is
at the tc hookpoint. This patch will extend this API at XDP
hookpoint. A XDP program can map the incoming packet to the
5-tuple parameter and invoke the API to find the corresponding
socket structure.

Signed-off-by: Nitin Hande <Nitin.Hande@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>