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 6eb5d2e0]

Change-Id: I70a700ec66e0ea8526a17137ba6cb5a3f2dce45f
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>