NFS: Deal correctly with attribute generation counter overflow

[ Upstream commit 9fdbfad1777cb4638f489eeb62d85432010c0031 ]

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>