scsi: target: Allow changing dbroot if there are no registered devices

scsi: target: Allow changing dbroot if there are no registered devices

The target driver prevents the users from changing the database root
directory if a target module like ib_srpt has been registered. This makes
it difficult for users to set their preferred database directory if the
module gets loaded during the system boot.

Let the users modify dbroot if there are no registered devices

Link: https://lore.kernel.org/r/20220328103940.19977-1-mlombard@redhat.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>