ipvs: bound LBLCR and LBLC cache growth

ip_vs_lblcr_new() and ip_vs_lblc_new() create cache entries for
every previously unseen destination address. The table max_size only
tells the periodic collector to reclaim entries after the cache has
already exceeded the limit. It does not reclaim entries that the
attacker continues to use.

Reject new cache entries once either table reaches max_size * 3 / 2.
The extra headroom lets the periodic collector catch up while the
existing scheduler fallback continues to use the selected destination
when cache creation fails. New traffic therefore stays serviceable
without growing the tables further.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Suggested-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>