[IPV6]: Make ndisc_dst_alloc() common for later use.

For later use, this patch is renaming ndisc_dst_alloc()
(and related function/structures) to icmp6_dst_alloc()
(and so on).  This patch also removing unused function-
pointer argument for it.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/ip6_fib.c

@@ -1464,7 +1464,7 @@ void fib6_run_gc(unsigned long dummy)
 	}
 	gc_args.more = 0;
 
-	ndisc_dst_gc(&gc_args.more);
+	icmp6_dst_gc(&gc_args.more);
 	fib6_clean_all(fib6_age, 0, NULL);
 
 	if (gc_args.more)