vti6: better validate user provided tunnel names

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

Fixes: ed1efb2a

 ("ipv6: Add support for IPsec virtual tunnel interfaces")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>