net: diag: Support SOCK_DESTROY for inet sockets.

This passes the SOCK_DESTROY operation to the underlying protocol
diag handler, or returns -EOPNOTSUPP if that handler does not
define a destroy operation.

Most of this patch is just renaming functions. This is not
strictly necessary, but it would be fairly counterintuitive to
have the code to destroy inet sockets be in a function whose name
starts with inet_diag_get.

[Backport of net-next 6eb5d2e08f071c05ecbe135369c9ad418826cab2]

Change-Id: Iee2c858bf11c48f54890b85b87821a2a2d7109e1
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>