sock_diag: Generalize requests cookies managements

The sk address is used as a cookie between dump/get_exact calls.
It will be required for unix socket sdumping, so move it from
inet_diag to sock_diag.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>