af_unix: Use designated initializers

Prepare to mark sensitive kernel structures for randomization by making
sure they're using designated initializers. These were identified during
allyesconfig builds of x86, arm, and arm64, and the initializer fixes
were extracted from grsecurity. In this case, NULL initialize with { }
instead of undesignated NULLs.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>