ip_gre: fix IFLA_MTU ignored on NEWLINK

It's safe to remove the setting of dev's needed_headroom and mtu in
__gre_tunnel_init, as discussed in [1], ip_tunnel_newlink can do it
properly.

Now Eric noticed that it could cover the mtu value set in do_setlink
when creating a ip_gre dev. It makes IFLA_MTU param not take effect.

So this patch is to remove them to make IFLA_MTU work, as in other
ipv4 tunnels.

  [1]: https://patchwork.ozlabs.org/patch/823504/

Fixes: c5441932

 ("GRE: Refactor GRE tunneling code.")
Reported-by: Eric Garver <e@erig.me>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>