[INET]: Move common fields from frag_queues in one place.

Introduce the struct inet_frag_queue in include/net/inet_frag.h
file and place there all the common fields from three structs:

 * struct ipq in ipv4/ip_fragment.c
 * struct nf_ct_frag6_queue in nf_conntrack_reasm.c
 * struct frag_queue in ipv6/reassembly.c

After this, replace these fields on appropriate structures with
this structure instance and fix the users to use correct names
i.e. hunks like

-    atomic_dec(&fq->refcnt);
+    atomic_dec(&fq->q.refcnt);

(these occupy most of the patch)

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>