gre: Correctly account for hardware header length.

gre: Correctly account for hardware header length.

On recent kernels there are two fields that we can use to keep track
of the overhead due to tunneling: hard_header_len and needed_headroom.
On older kernels everything got shoved into hard_header_len and the
actual header length got lost, which caused MTU calculations to be
incorrect.  Since we know the device type we can figure out what the
header length should be.