ipv4: Disable route secret interval on zero interval

Let me first state that disabling the route cache hash rebuild
should not be done without extensive analysis on the risk profile
and careful deliberation.

However, there are times when this can be done safely or for
testing.  For example, when you have mechanisms for ensuring
that offending parties do not exist in your network.

This patch lets the user disable the rebuild if the interval is
set to zero.  This also incidentally fixes a divide-by-zero error
with name-spaces.

In addition, this patch makes the effect of an interval change
immediate rather than it taking effect at the next rebuild as
is currently the case.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>