[PATCH] libata: clear sdev->locked on door lock failure

SCSI EH locks door if sdev->locked is set.  Sometimes door lock
command fails continuously (e.g. when medium is not present) and as
libata uses EH to acquire sense data, this easily creates a loop where
a failed lock door invokes EH and EH issues lock door on completion.

This patch clears sdev->locked on door lock failure to break this
loop.  This problem has been spotted and diagnosed by Unicorn Chang
<uchang@tw.ibm.com>.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>