ipv4: consolidate the VIFF_TUNNEL handling in ipmr_new_tunnel

Also move the dev_set_allmulti call and the error handling into the
ioctl helper.  This allows reusing already looked up tunnel_dev pointer
and the set up argument structure for the deletion in the error handler.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>