MFC r328250: MFV r328249:
8641 "zpool clear" and "zinject" don't work on "spare" or "replacing" vdevs

illumos/illumos-gate@2ba5f978a4f9b02da9db1b8cdd9ea5498eb00ad9

https://www.illumos.org/issues/8641:
"zpool clear" and "zinject -d" can both operate on specific vdevs, either
leaf or interior. However, due to an oversight, neither works on a "spare"
or "replacing" vdev. For example:

sudo zpool create foo raidz1 c1t5000CCA000081D61d0 c1t5000CCA000186235d0 spare c
1t5000CCA000094115d0
sudo zpool replace foo c1t5000CCA000186235d0 c1t5000CCA000094115d0
$ zpool status foo pool: foo
state: ONLINE
scan: resilvered 81.5K in 0h0m with 0 errors on Fri Sep 8 10:53:03 2017
config:

NAME STATE READ WRITE CKSUM

foo                          ONLINE       0     0     0
  raidz1-0                   ONLINE       0     0     0
    c1t5000CCA000081D61d0    ONLINE       0     0     0
    spare-1                  ONLINE       0     0     0
      c1t5000CCA000186235d0  ONLINE       0     0     0
      c1t5000CCA000094115d0  ONLINE       0     0     0
spares
  c1t5000CCA000094115d0      INUSE     currently in use

$ sudo zinject -d spare-1 -A degrade foo
cannot find device 'spare-1' in pool 'foo'
$ sudo zpool clear foo spare-1
cannot clear errors for spare-1: no such device in pool

Even though there was nothing to clear, those commands shouldn't have
reported an error. by contrast, trying to clear "raidz1-0" works just fine:
$ sudo zpool clear foo raidz1-0

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Alan Somers <asomers@gmail.com>