fou: fix some member types in guehdr

guehdr struct is used to build or parse gue packets, which
are always in big endian. It's better to define all guehdr
members as __beXX types.

Also, in validate_gue_flags it's not good to use a __be32
variable for both Standard flags(__be16) and Private flags
(__be32), and pass it to other funcions.

This patch could fix a bunch of sparse warnings from fou.

Fixes: 5024c33a ("gue: Add infrastructure for flags and options")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>