ip_fragment: fix subtracting PPPOE_SES_HLEN from mtu twice

6c79bf0f subtracts PPPOE_SES_HLEN from mtu at
the front of ip_fragment(). So the later subtraction should be removed. The
MTU of 802.1q is also 1500, so MTU should not be changed.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Bart De Schuymer <bdschuym@pandora.bo>
----
 net/ipv4/ip_output.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)
Signed-off-by: Bart De Schuymer <bdschuym@pandora.bo>
Signed-off-by: David S. Miller <davem@davemloft.net>