ip6_tunnel: better validate user provided tunnel names

[ Upstream commit db7a65e3 ]

Use valid_name() to make sure user does not provide illegal
device name.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>