md devices may leave remnants in /sys/class/block after removal

1. root@x1:~# cat /proc/version_signature
Ubuntu 4.4.0-72.93-generic 4.4.49

2. attached

3. # lsb_release -rd
Description: Ubuntu 16.04.2 LTS
Release: 16.04

4. # apt-cache policy linux-image-`uname -r`
linux-image-4.4.0-72-generic:
  Installed: 4.4.0-72.93
  Candidate: 4.4.0-72.93
  Version table:
 *** 4.4.0-72.93 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status

5. After stopping an md device, the sysfs and /dev entry should be removed:

mdadm --stop /dev/md0 && [ "`test -e /sys/class/block/md0`" = "1" ]; echo $rc
should return 0;

6. /dev/md0 is gone, but /sys/class/block/md0 remains

I believe this is related to this thread here:

https://www.spinics.net/lists/raid/msg54859.html

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-72-generic 4.4.0-72.93
ProcVersionSignature: Ubuntu 4.4.0-72.93-generic 4.4.49
Uname: Linux 4.4.0-72-generic x86_64
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Apr 12 21:55 seq
 crw-rw---- 1 root audio 116, 33 Apr 12 21:55 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
Date: Thu Apr 13 14:32:17 2017
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb: Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
PciMultimedia:

ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB:

ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-72-generic root=UUID=4c974bc3-8fe8-41b5-a563-23b2dcf974c9 ro console=tty1 console=ttyS0
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-72-generic N/A
 linux-backports-modules-4.4.0-72-generic N/A
 linux-firmware N/A
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: Ubuntu-1.8.2-1ubuntu2
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-yakkety
dmi.modalias: dmi:bvnSeaBIOS:bvrUbuntu-1.8.2-1ubuntu2:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-yakkety:cvnQEMU:ct1:cvrpc-i440fx-yakkety:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-yakkety
dmi.sys.vendor: QEMU