UDP sockets are hashed in a 128 slots hash table.

This hash table is protected by *one* rwlock.

This rwlock is readlocked each time an incoming UDP message is handled.

This rwlock is writelocked each time a socket must be inserted in
hash table (bind time), or deleted from this table (close time)

This is not scalable on SMP machines :

1) Even in read mode, lock() and unlock() are atomic operations and
  must dirty a contended cache line, shared by all cpus.

2) A writer might be starved if many readers are 'in flight'. This can
  happen on a machine with some NIC receiving many UDP messages. User
  process can be delayed a long time at socket creation/dismantle time.

This patch prepares RCU migration, by introducing 'struct udp_table
and struct udp_hslot', and using one rwlock per chain, to reduce
contention on central rwlock.

Introducing one rwlock per chain reduces latencies, for port
randomization on heavily loaded UDP servers.

Some cleanups were done to ease review of following patch.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 include/net/sock.h    |    2
 include/net/udp.h     |   25 ++--
 include/net/udplite.h |    2
 net/ipv4/udp.c        |  208 +++++++++++++++++++++++-----------------
 net/ipv4/udp_impl.h   |    4
 net/ipv4/udplite.c    |   13 +-
 net/ipv6/udp.c        |  112 +++++++++++----------
 net/ipv6/udp_impl.h   |    4
 net/ipv6/udplite.c    |    8 -
 9 files changed, 214 insertions(+), 164 deletions(-)