ip6tnl: fix sparse warnings in ip6_tnl_netlink_parms()

This change fixes a sparse warning triggered by casting the flowinfo from
netlink messages in an u32 instead of be32. This change corrects that in order
to resolve the sparse warning.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>