writeback: add name to backing_dev_info

This enables us to track who does what and print info. Its main use
is catching dirty inodes on the default_backing_dev_info, so we can
fix that up.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index e83be2e..15831d5 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1352,6 +1352,7 @@
 {
 	int err;
 
+	bdi->name = "btrfs";
 	bdi->capabilities = BDI_CAP_MAP_COPY;
 	err = bdi_init(bdi);
 	if (err)