change the comment of vti6_ioctl

The comment of vti6_ioctl() is wrong. which use vti6_tnl_ioctl
instead of vti6_ioctl.

Signed-off-by: Sun Lianwen <sunlw.fnst@cn.fujitsu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>