[SCSI] Fix SCSI tape oops at module removal

Removing the SCSI tape module results in an oops in class_device_destroy if
any devices are present. The patch at the end of this message fixes the bug
by moving class_destroy() later in exit_st() so that the class still exists
when devices are removed. (The bug is old but class_simple_device_remove() did
nothing when the class did not exist.)

The patch also fixes a "class leak" in init_st() error path.

I would like to get this into 2.6.13 but it may be too late?

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>