sched/task_stack: Add missing const qualifier to end_of_stack()

Add missing const qualifier to the non-CONFIG_THREAD_INFO_IN_TASK
version of end_of_stack() to match the CONFIG_THREAD_INFO_IN_TASK
version. Fixes a warning with CONFIG_KSTACK_ERASE=y on archs that don't
select THREAD_INFO_IN_TASK (such as LoongArch):

  error: passing 'const struct task_struct *' to parameter of type 'struct task_struct *' discards qualifiers

The stackleak_task_low_bound() function correctly uses a const task
parameter, but the legacy end_of_stack() prototype didn't like that.

Build tested on loongarch (with CONFIG_KSTACK_ERASE=y) and m68k
(with CONFIG_DEBUG_STACK_USAGE=y).

Fixes: a45728fd ("LoongArch: Enable HAVE_ARCH_STACKLEAK")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20250726004313.GA3650901@ax162


Cc: Youling Tang <tangyouling@kylinos.cn>
Cc: Huacai Chen <chenhuacai@loongson.cn>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <kees@kernel.org>