gre: Use inner mac length when computing tunnel length
gre: Use inner mac length when computing tunnel length

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 <0786393240831fc3b5d6ed63658cfb1f3b1e6c40@google.com>
Acked-by: Alexander Duyck <af56634f9be8457194a3461ec96c497f138af4ff@redhat.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs