ipv6: move IPV6_TCLASS_SHIFT into ipv6.h and define a helper

Two places defined IPV6_TCLASS_SHIFT, so we should move it into ipv6.h,
and use this macro as possible. And define ip6_tclass helper to return
tclass

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/fib6_rules.c b/net/ipv6/fib6_rules.c
index 3fd0a57..b4d5e1d 100644
--- a/net/ipv6/fib6_rules.c
+++ b/net/ipv6/fib6_rules.c
@@ -169,7 +169,7 @@
 			return 0;
 	}
 
-	if (r->tclass && r->tclass != ((ntohl(fl6->flowlabel) >> 20) & 0xff))
+	if (r->tclass && r->tclass != ip6_tclass(fl6->flowlabel))
 		return 0;
 
 	return 1;