scsi: fix memory leak of sdpk on when gd fails to allocate

On an allocation failure of gd, the current exit path is via
out_free_devt which leaves sdpk still allocated and hence it gets
leaked. Fix this by correcting the order of resource free'ing with a
change in the error exit path labels.

Detected by CoverityScan, CID#1399519 ("Resource Leak")

Fixes: 0dba1314 ("scsi, block: fix duplicate bdi name registration crashes")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Bart Van Assche <Bart.VanAssche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>