tun: Return -EINVAL if neither IFF_TUN nor IFF_TAP is set.

After commit 2b980dbd
("lsm: Add hooks to the TUN driver") tun_set_iff doesn't
return -EINVAL though neither IFF_TUN nor IFF_TAP is set.

Signed-off-by: Kusanagi Kouichi <slash@ma.neweb.ne.jp>
Reviewed-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>