ipv6: Fix endianess warning in ip6_flow_hdr().

Commit 3e4e4c1f ("ipv6: Introduce ip6_flow_hdr() to fill version,
tclass and flowlabel.) uses ntohl(), which should be htonl().

Found by Fengguang Wu <fengguang.wu@intel.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>