NFS: Deal correctly with attribute generation counter overflow

stable inclusion
from linux-4.19.191
commit e6681459e47a09fe61eac157aff93f2326d64025

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

[ Upstream commit 9fdbfad1 ]

We need to use unsigned long subtraction and then convert to signed in
order to deal correcly with C overflow rules.

Fixes: f5062003 ("NFS: Set an attribute barrier on all updates")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>