s390/fpu: make use of __uninitialized macro

Code sections in s390 specific kernel code which use floating point or
vector registers all come with a 520 byte stack variable to save already in
use registers, if required.

With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled this variable
will always be initialized on function entry in addition to saving register
contents, which contradicts the intention (performance improvement) of such
code sections.

Therefore provide a DECLARE_KERNEL_FPU_ONSTACK() macro which provides
struct kernel_fpu variables with an __uninitialized attribute, and convert
all existing code to use this.

This way only this specific type of stack variable will not be initialized,
regardless of config options.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240205154844.3757121-3-hca@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/sysinfo.c

@@ -426,9 +426,9 @@ subsys_initcall(create_proc_service_level);
  */
 void s390_adjust_jiffies(void)
 {
+	DECLARE_KERNEL_FPU_ONSTACK(fpu);
 	struct sysinfo_1_2_2 *info;
 	unsigned long capability;
-	struct kernel_fpu fpu;
 
 	info = (void *) get_zeroed_page(GFP_KERNEL);
 	if (!info)