scsi: fix device handler detach oops

This fixes a regression caused by commit 1d5203 ("scsi: handle more device
handler setup/teardown in common code").

The bug is that the alua detach() callout will try to access the
sddev->scsi_dh_data, but we have already set it to NULL. This patch
moves the clearing of that field to after detach() is called.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>