btrfs: support notification upon degraded mounts
Shell provides notification upon mdadm degraded array assembly (I'm not sure what conditions need to be met).
When using Btrfs raid1 and degraded mount is required, there should be parity with this notification policy/behavior. This could be as simple as checking for 'degraded' mount option for the file system.
Related: https://github.com/kdave/btrfs-progs/issues/264 https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/message/4HGKNDKW3VXWO3JX2KB3SVB6WY7HPAVQ/
Note: Btrfs doesn't have a concept of faulty devices like mdadm. If mount is normal, and a drive subsequently fails, the only available information at the moment will be many kernel messages: either block driver or perhaps Btrfs alone, depending on the nature of the problem. Maybe smartd will detect the failure too but it's less reliable.