tun: Don't add sysfs attributes to devices without sysfs directories

tun: Don't add sysfs attributes to devices without sysfs directories

This applies to 2.6.32 *only*.  It has not been applied upstream since
the limitation no longer exists.

Prior to Linux 2.6.35, net devices outside the initial net namespace
did not have sysfs directories.  Attempting to add attributes to
them will trigger a BUG().

Reported-and-tested-by: Russell Stuart <russell-debian@stuart.id.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>