fib_hash: RCU conversion phase 2

Get rid of fib_hash_lock rwlock.

The fn_zone hash table resize is the noticeable part of this patch.

I added a seqlock per fn_zone, so that readers can restart their lookup
in the (very rare) case a writer expanded the hash table.

Add rcu heads in fib_alias and fib_node, use call_rcu() to defer their
freeing, and use appropriate _rcu list manipulations.

Stress test (160.000.000 udp frames sent, IP route cache disabled to
mimic DDOS attack, FIB_HASH)

Before:
real	0m41.191s
user	0m13.137s
sys	8m55.241s

After:
real	0m38.091s
user	0m13.189s
sys	7m53.018s
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_lookup.h

@@ -12,9 +12,7 @@ struct fib_alias {
 	u8			fa_type;
 	u8			fa_scope;
 	u8			fa_state;
-#ifdef CONFIG_IP_FIB_TRIE
 	struct rcu_head		rcu;
-#endif
 };
 
 #define FA_S_ACCESSED	0x01