[NETFILTER]: nf_conntrack: use hashtable for expectations

Currently all expectations are kept on a global list that

- needs to be searched for every new conncetion
- needs to be walked for evicting expectations when a master connection
  has reached its limit
- needs to be walked on connection destruction for connections that
  have open expectations

This is obviously not good, especially when considering helpers like
H.323 that register *lots* of expectations and can set up permanent
expectations, but it also allows for an easy DoS against firewalls
using connection tracking helpers.

Use a hashtable for expectations to avoid incurring the search overhead
for every new connection. The default hash size is 1/256 of the conntrack
hash table size, this can be overriden using a module parameter.

This patch only introduces the hash table for expectation lookups and
keeps other users to reduce the noise, the following patches will get
rid of it completely.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>