sgi-xp: Use designated initializers

Prepare to mark sensitive kernel structures for randomization by making
sure they're using designated initializers. In this case, no initializers
are needed (they can be NULL initialized and callers adjusted to check
for NULL, which is more efficient than an indirect call).

Cc: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christoph Hellwig <hch@infradead.org>