bsg: fix the deadlock on discarding done commands

The previous commit introduced a deadlock in discarding commands,
because we forget to unlock the bd spinlock.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/bsg.c b/block/bsg.c
index 2f78d7d..5f4abc9 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -471,6 +471,7 @@
 			spin_unlock_irq(&bd->lock);
 			break;
 		}
+		spin_unlock_irq(&bd->lock);
 
 		bc = bsg_get_done_cmd(bd);
 		if (IS_ERR(bc))