gre: Set inner mac header in gro complete

[ Upstream commit 6fb2a756739aa507c1fd5b8126f0bfc2f070dc46 ]

Set the inner mac header to point to the GRE payload when
doing GRO. This is needed if we proceed to send the packet
through GRE GSO which now uses the inner mac header instead
of inner network header to determine the length of encapsulation
headers.

Fixes: 14051f04 ("gre: Use inner mac length when computing tunnel length")
Reported-by: Wolfgang Walter <linux@stwm.de>
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>