BACKPORT [FROMLIST] New getsockopt option to get socket cookie

Cherry-pick from linux net-next branch commit
http://patchwork.ozlabs.org/patch/747590/

Introduce a new getsockopt operation to retrieve the socket cookie
for a specific socket based on the socket fd.  It returns a unique
non-decreasing cookie for each socket.
Tested: https://android-review.googlesource.com/#/c/358163/



Test: Unit test added in kernel/tests

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>