gre: Use inner mac length when computing tunnel length

gre: Use inner mac length when computing tunnel length

[ Upstream commit 14051f0452a2c26a3f4791e6ad6a435e8f1945ff ]

Currently, skb_inner_network_header is used but this does not account
for Ethernet header for ETH_P_TEB. Use skb_inner_mac_header which
handles TEB and also should work with IP encapsulation in which case
inner mac and inner network headers are the same.

Tested: Ran TCP_STREAM over GRE, worked as expected.

Signed-off-by: Tom Herbert <therbert@google.com>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>