driver: tun: Move tun check into the block of TUNSETIFF condition check

When cmd is TUNSETIFF and tun is not null, the original codes go ahead,
then reach the default case of switch(cmd) and set the ret is -EINVAL.
It is not clear for readers.

Now move the tun check into the block of TUNSETIFF condition check, and
return -EEXIST instead of -EINVAL when the tfile already owns one tun.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>