There are some common codes for stack checking, so factors it out into
the function on_stack().

No functional change.
Signed-off-by: Yunfeng Ye <yeyunfeng@huawei.com>
Link: https://lore.kernel.org/r/07b3b0e6-3f58-4fed-07ea-7d17b7508948@huawei.comSigned-off-by: Will Deacon <will@kernel.org>