net: RCU lookups for UDP, DCCP and TCP protocol

Hi all

Here is a serie of three patches (based on net-next-2.6), to continue work with RCU on UDP/TCP/DCCP stacks

Many thanks for all usefull reviews and comments, especially from Paul and Corey.

1) Introduce hlist_nulls variant of hlist

hlist uses NULL value to finish a chain.
hlist_nulls variant use the low order bit set to 1 to signal an end marker. This allows to store many different end markers, so that some RCU lockless algos (used in TCP/UDP stack for example) can save some memory barriers in fast paths.

2) Use hlist_nulls in UDP RCU code

This is a straightforward patch, using hlist_nulls infrastructure. RCU-ification already done on UDP two weeks ago, so hlist_nulls permits us to avoid some memory barriers, both at lookup time and delete time. Patch is large because it adds new macros to include/net/sock.h. These macros will be used by TCP & DCCP too.

3) Convert TCP & DCCP hash tables to use RCU & hlist_nulls

RCU was added to UDP lookups, using a fast infrastructure :
- sockets kmem_cache use SLAB_DESTROY_BY_RCU and dont pay the price of call_rcu() at freeing time.
- hlist_nulls permits to use few memory barriers.

This patch uses same infrastructure for TCP/DCCP established and timewait sockets.

Thanks to SLAB_DESTROY_BY_RCU, no slowdown for applications using short lived TCP connections. A followup patch, converting rwlocks to spinlocks will even speedup this case.

__inet_lookup_established() is pretty fast now we dont have to dirty a contended cache line (read_lock/read_unlock)

Only established and timewait hashtable are converted to RCU
(bind table and listen table are still using traditional locking)