scsi_dh: don't allow to detach device handlers at runtime

The I/O submission and completion paths call into the device handler
without any synchronization agains detachment.  So disallow detaching
device handlers at runtime.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>