ipv4: Limit size of route cache hash table

Right now we have no upper limit on the size of the route cache hash table.
On a 128GB POWER6 box it ends up as 32MB:

    IP route cache hash table entries: 4194304 (order: 9, 33554432 bytes)

It would be nice to cap this for memory consumption reasons, but a massive
hashtable also causes a significant spike when measuring OS jitter.

With a 32MB hashtable and 4 million entries, rt_worker_func is taking
5 ms to complete. On another system with more memory it's taking 14 ms.
Even though rt_worker_func does call cond_sched() to limit its impact,
in an HPC environment we want to keep all sources of OS jitter to a minimum.

With the patch applied we limit the number of entries to 512k which
can still be overriden by using the rt_entries boot option:

    IP route cache hash table entries: 524288 (order: 6, 4194304 bytes)

With this patch rt_worker_func now takes 0.460 ms on the same system.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>