vdev_disk: don't touch vbio after its handed off to the kernel

After IO is unplugged, it may complete immediately and vbio_completion
be called on interrupt context. That may interrupt or deschedule our
task. If its the last bio, the vbio will be freed. Then, we get
rescheduled, and try to write to freed memory through vbio->.

This patch just removes the the cleanup, and the corresponding assert.
These were leftovers from a previous iteration of vbio_submit() and were
always "belt and suspenders" ops anyway, never strictly required.

Reported-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes: openzfs#16045
Closes: openzfs#16050

module/os/linux/zfs/vdev_disk.c

@@ -755,8 +755,6 @@ vbio_fill_cb(struct page *page, size_t off, size_t len, void *priv)
 static void
 vbio_submit(vbio_t *vbio, abd_t *abd, uint64_t size)
 {
-	ASSERT(vbio->vbio_bdev);
-
 	/*
 	 * We plug so we can submit the BIOs as we go and only unplug them when
 	 * they are fully created and submitted. This is important; if we don't
@@ -774,12 +772,15 @@ vbio_submit(vbio_t *vbio, abd_t *abd, uint64_t size)
 	vbio->vbio_bio->bi_end_io = vbio_completion;
 	vbio->vbio_bio->bi_private = vbio;
 
+	/*
+	 * Once submitted, vbio_bio now owns vbio (through bi_private) and we
+	 * can't touch it again. The bio may complete and vbio_completion() be
+	 * called and free the vbio before this task is run again, so we must
+	 * consider it invalid from this point.
+	 */
 	vdev_submit_bio(vbio->vbio_bio);
 
 	blk_finish_plug(&plug);
-
-	vbio->vbio_bio = NULL;
-	vbio->vbio_bdev = NULL;
 }
 
 /* IO completion callback */