[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>

net/ipv4/proc.c

@@ -70,8 +70,8 @@ static int sockstat_seq_show(struct seq_file *seq, void *v)
 	seq_printf(seq, "UDP: inuse %d\n", fold_prot_inuse(&udp_prot));
 	seq_printf(seq, "UDPLITE: inuse %d\n", fold_prot_inuse(&udplite_prot));
 	seq_printf(seq, "RAW: inuse %d\n", fold_prot_inuse(&raw_prot));
-	seq_printf(seq,  "FRAG: inuse %d memory %d\n", ip_frag_nqueues,
-		   atomic_read(&ip_frag_mem));
+	seq_printf(seq,  "FRAG: inuse %d memory %d\n",
+			ip_frag_nqueues(), ip_frag_mem());
 	return 0;
 }