BACKPORT: UPSTREAM: Add a helper function to get socket cookie in eBPF

Cherrypick from commit: 91b8270f2a4d1d9b268de90451cdca63a70052d6

Retrieve the socket cookie generated by sock_gen_cookie() from a sk_buff
with a known socket. Generates a new cookie if one was not yet set.If
the socket pointer inside sk_buff is NULL, 0 is returned. The helper
function coud be useful in monitoring per socket networking traffic
statistics and provide a unique socket identifier per namespace.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Bug: 30950746
Change-Id: I95918dcc3ceffb3061495a859d28aee88e3cde3c
Signed-off-by: Chatur27 <jasonbright2709@gmail.com>