ip_gre: Set needed_headroom dynamically again

ip_gre: Set needed_headroom dynamically again

Now that all needed_headroom users have been fixed up so that
we can safely increase needed_headroom, this patch restore the
dynamic update of needed_headroom.

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