ubsan: Fix panic from test_ubsan_out_of_bounds

Running lib_ubsan.ko on arm64 (without CONFIG_UBSAN_TRAP) panics the
kernel:

[ 31.616546] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: test_ubsan_out_of_bounds+0x158/0x158 [test_ubsan]
[ 31.646817] CPU: 3 UID: 0 PID: 179 Comm: insmod Not tainted 6.15.0-rc2 #1 PREEMPT
[ 31.648153] Hardware name: linux,dummy-virt (DT)
[ 31.648970] Call trace:
[ 31.649345] show_stack+0x18/0x24 (C)
[ 31.650960] dump_stack_lvl+0x40/0x84
[ 31.651559] dump_stack+0x18/0x24
[ 31.652264] panic+0x138/0x3b4
[ 31.652812] __ktime_get_real_seconds+0x0/0x10
[ 31.653540] test_ubsan_load_invalid_value+0x0/0xa8 [test_ubsan]
[ 31.654388] init_module+0x24/0xff4 [test_ubsan]
[ 31.655077] do_one_initcall+0xd4/0x280
[ 31.655680] do_init_module+0x58/0x2b4

That happens because the test corrupts other data in the stack:
400: d5384108 mrs x8, sp_el0
404: f9426d08 ldr x8, [x8, #1240]
408: f85f83a9 ldur x9, [x29, #-8]
40c: eb09011f cmp x8, x9
410: 54000301 b.ne 470 <test_ubsan_out_of_bounds+0x154> // b.any

As there is no guarantee the compiler will order the local variables
as declared in the module:
volatile char above[4] = { }; /* Protect surrounding memory. */
volatile int arr[4];
volatile char below[4] = { }; /* Protect surrounding memory. */

There is another problem where the out-of-bound index is 5 which is larger
than the extra surrounding memory for protection.

So, use a struct to enforce the ordering, and fix the index to be 4.
Also, remove some of the volatiles and rely on OPTIMIZER_HIDE_VAR()

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20250415203354.4109415-1-smostafa@google.com
Signed-off-by: Kees Cook <kees@kernel.org>