Cancel initialize and TRIM before vdev_metaslab_fini()

Any running 'zpool initialize' or TRIM must be cancelled prior
to the vdev_metaslab_fini() call in spa_vdev_remove_log() which
will unload the metaslabs and set ms->ms_group == NULL.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8602
Closes #9751