inet_diag: use jiffies_delta_to_msecs()

[ Upstream commit 3828a93f ]

Use jiffies_delta_to_msecs() to avoid reporting 'infinite'
timeouts and to cleanup code.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>