add jiffies_delta_to_msecs() helper func to calculate the delta between
two times and eventually 0 if negative.
Suggested-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NMatteo Croce <mcroce@redhat.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSimon Horman <horms@verge.net.au>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>