btrfs: Update existing btrfs_device for renaming device

When we scan devices in a multi-device filesystem, we memorize the original
name.  If the device gets a new name, later scans don't update the
in-kernel structures related to it, and we're not able to mount the
filesystem.

This patch updates device name during scaning.

Signed-off-by: TARUISI Hiroaki <taruishi.hiroak@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>