jiffies: add utility function to calculate delta in ms

add jiffies_delta_to_msecs() helper func to calculate the delta between
two times and eventually 0 if negative.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>