[SCSI] sd: fix medium-removal bug

Commit 409f3499 (scsi/sd: remove big
kernel lock) introduced a bug in the sd_release routine.  Medium
removal should be allowed when the number of open file references
drops to 0, not when it becomes non-zero.

This patch (as1414) adjusts the test to fix the bug.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>