s390/ptrace: Fix pointer dereferencing in regs_get_kernel_stack_nth()

The recent change which added READ_ONCE_NOCHECK() to read the nth entry
from the kernel stack incorrectly dropped dereferencing of the stack
pointer in order to read the requested entry.

In result the address of the entry is returned instead of its content.

Dereference the pointer again to fix this.

Reported-by: Will Deacon <will@kernel.org>
Closes: https://lore.kernel.org/r/20250612163331.GA13384@willie-the-truck


Fixes: d93a855c ("s390/ptrace: Avoid KASAN false positives in regs_get_kernel_stack_nth()")
Cc: stable@vger.kernel.org
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>