net: diag: Add the ability to destroy a socket.

This patch adds a SOCK_DESTROY operation, a destroy function
pointer to sock_diag_handler, and a diag_destroy function
pointer.  It does not include any implementation code.

[Backport of net-next 64be0aed59ad519d6f2160868734f7e278290ac1]

Change-Id: I24f4a157e0a2cdb267317920aa230cc6528ca072
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>