task_stack: Fix end_of_stack() for architectures with upwards-growing stack

stable inclusion
from linux-4.19.218
commit ca6b1b3e032cf6ae2a61303eba04848b10bd6efc

--------------------------------

[ Upstream commit 9cc2fa4f ]

The function end_of_stack() returns a pointer to the last entry of a
stack. For architectures like parisc where the stack grows upwards
return the pointer to the highest address in the stack.

Without this change I faced a crash on parisc, because the stackleak
functionality wrote STACKLEAK_POISON to the lowest address and thus
overwrote the first 4 bytes of the task_struct which included the
TIF_FLAGS.

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>