The GRE header length should be subtracted when the tunnel MTU is
calculated. This just corrects for the associativity change
introduced by commit 42aa916265d740d66ac1f17290366e9494c884c2
("gre: Move MTU setting out of ipgre_tunnel_bind_dev").
Signed-off-by: Tom Goff <thomas.goff@boeing.com> Signed-off-by: David S. Miller <davem@davemloft.net>