[SCSI] bsg: fix oops on remove

If you do a modremove of any sas driver, you run into an oops on
shutdown when the host is removed (coming from the host bsg device).
The root cause seems to be that there's a use after free of the
bsg_class_device:  In bsg_kref_release_function, this is used (to do a
put_device(bcg->parent) after bcg->release has been called.  In sas (and
possibly many other things) bcd->release frees the queue which contains
the bsg_class_device, so we get a put_device on unreferenced memory.
Fix this by taking a copy of the pointer to the parent before releasing
bsg.

Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

block/bsg.c

@@ -709,11 +709,12 @@ static void bsg_kref_release_function(struct kref *kref)
 {
 	struct bsg_class_device *bcd =
 		container_of(kref, struct bsg_class_device, ref);
+	struct device *parent = bcd->parent;
 
 	if (bcd->release)
 		bcd->release(bcd->parent);
 
-	put_device(bcd->parent);
+	put_device(parent);
 }
 
 static int bsg_put_device(struct bsg_device *bd)