[INET]: Collect frag queues management objects together

There are some objects that are common in all the places
which are used to keep track of frag queues, they are:

 * hash table
 * LRU list
 * rw lock
 * rnd number for hash function
 * the number of queues
 * the amount of memory occupied by queues
 * secret timer

Move all this stuff into one structure (struct inet_frags)
to make it possible use them uniformly in the future. Like
with the previous patch this mostly consists of hunks like

-    write_lock(&ipfrag_lock);
+    write_lock(&ip4_frags.lock);

To address the issue with exporting the number of queues and
the amount of memory occupied by queues outside the .c file
they are declared in, I introduce a couple of helpers.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ipv6.h b/include/net/ipv6.h
index 31b3f1b..77cdab3 100644
--- a/include/net/ipv6.h
+++ b/include/net/ipv6.h
@@ -252,8 +252,8 @@
 
 extern int ipv6_opt_accepted(struct sock *sk, struct sk_buff *skb);
 
-extern int ip6_frag_nqueues;
-extern atomic_t ip6_frag_mem;
+int ip6_frag_nqueues(void);
+int ip6_frag_mem(void);
 
 #define IPV6_FRAG_TIMEOUT	(60*HZ)		/* 60 seconds */