btrfs: check-integrity: Fix NULL pointer dereference for degraded mount

Commit f8f84b2d ("btrfs: index check-integrity state hash by a dev_t")
changed how btrfsic indexes device state.

Now we need to access device->bdev->bd_dev, while for degraded mount
it's completely possible to have device->bdev as NULL, thus it will
trigger a NULL pointer dereference at mount time.

Fix it by checking if the device is degraded before accessing
device->bdev->bd_dev.

There are a lot of other places accessing device->bdev->bd_dev, however
the other call sites have either checked device->bdev, or the
device->bdev is passed from btrfsic_map_block(), so it won't cause harm.

Fixes: f8f84b2d ("btrfs: index check-integrity state hash by a dev_t")
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/check-integrity.c

@@ -1539,7 +1539,12 @@ static int btrfsic_map_block(struct btrfsic_state *state, u64 bytenr, u32 len,
 	}
 
 	device = multi->stripes[0].dev;
-	block_ctx_out->dev = btrfsic_dev_state_lookup(device->bdev->bd_dev);
+	if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state) ||
+	    !device->bdev || !device->name)
+		block_ctx_out->dev = NULL;
+	else
+		block_ctx_out->dev = btrfsic_dev_state_lookup(
+							device->bdev->bd_dev);
 	block_ctx_out->dev_bytenr = multi->stripes[0].physical;
 	block_ctx_out->start = bytenr;
 	block_ctx_out->len = len;