add a new ->ndo_tunnel_ctl method to avoid a few set_fs calls v2

[Christoph Hellwig]

Hi Dave,

both the ipv4 and ipv6 code have an ioctl each that can be used to create
a tunnel using code that doesn't live in the core kernel or ipv6 module.
Currently they call ioctls on the tunnel devices to create these, for
which the code needs to override the address limit, which is a "feature"
I plan to get rid of.

Instead this patchset adds a new ->ndo_tunnel_ctl that can be used for
the tunnel configuration using struct ip_tunnel_parm.  The method is
either invoked from a helper that does the uaccess and can be wired up
as ndo_do_ioctl method, or directly from the magic IPV4/6 ioctls that
create tunnels with kernel space arguments.

Changes since v2:
 - properly propagate errors in ipip6_tunnel_prl_ctl