ipv6: Support multipath hashing on inner IP pkts

Make the same support as commit 363887a2cd ("ipv4: Support multipath
hashing on inner IP pkts for GRE tunnel") for outer IPv6. The hashing
considers both IPv4 and IPv6 pkts when they are tunneled by IPv6 GRE.

Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1473,6 +1473,7 @@ fib_multipath_hash_policy - INTEGER
 	Possible values:
 	0 - Layer 3 (source and destination addresses plus flow label)
 	1 - Layer 4 (standard 5-tuple)
+	2 - Layer 3 or inner Layer 3 if present
 
 anycast_src_echo_reply - BOOLEAN
 	Controls the use of anycast addresses as source addresses for ICMPv6