btrfs: false BUG_ON when degraded

In degraded mode the struct btrfs_device of missing devs don't have
device->name set. A kstrdup of NULL correctly returns NULL. Don't
BUG in this case.

Signed-off-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>