decnet: Use container_of() for struct dn_neigh casts

Clang's structure layout randomization feature gets upset when it sees
struct neighbor (which is randomized) cast to struct dn_neigh:

net/decnet/dn_route.c:1123:15: error: casting from randomized structure pointer type 'struct neighbour *' to 'struct dn_neigh *'
			gateway = ((struct dn_neigh *)neigh)->addr;
				   ^

Update all the open-coded casts to use container_of() to do the conversion
instead of depending on strict member ordering.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202205041247.WKBEHGS5-lkp@intel.com


Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Yajun Deng <yajun.deng@linux.dev>
Cc: Zheng Yongjun <zhengyongjun3@huawei.com>
Cc: Bill Wendling <morbo@google.com>
Cc: linux-decnet-user@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220508102217.2647184-1-keescook@chromium.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>