nfsd: helper for laundromat expiry calculations

[ Upstream commit 7f7e7a40 ]

We do this same logic repeatedly, and it's easy to get the sense of the
comparison wrong.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>