gre: Fix MTU calculation for bound GRE tunnels

[ Upstream commit 8cdb0456 ]

The GRE header length should be subtracted when the tunnel MTU is
calculated.  This just corrects for the associativity change
introduced by commit 42aa9162
("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>