gre: Use needed_headroom

Now that we have dev->needed_headroom, we can use it instead of
having a bogus dev->hard_header_len.  This also allows us to
include dev->hard_header_len in the MTU computation so that when
we do have a meaningful hard_harder_len in future it is included
automatically in figuring out the MTU.

Incidentally, this fixes a bug where we ignored the needed_headroom
field of the underlying device in calculating our own hard_header_len.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>