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 <[email protected]>
Closes: https://lore.kernel.org/r/20250612163331.GA13384@willie-the-truck
Fixes: d93a855 ("s390/ptrace: Avoid KASAN false positives in regs_get_kernel_stack_nth()")
Cc: [email protected]
Reviewed-by: Alexander Gordeev <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>

Author: hcahca

arch/s390/include/asm/ptrace.h

@@ -265,7 +265,7 @@ static __always_inline unsigned long regs_get_kernel_stack_nth(struct pt_regs *r
 	addr = kernel_stack_pointer(regs) + n * sizeof(long);
 	if (!regs_within_kernel_stack(regs, addr))
 		return 0;
-	return READ_ONCE_NOCHECK(addr);
+	return READ_ONCE_NOCHECK(*(unsigned long *)addr);
 }
 
 /**