The following test fails when the md_check_recovery() event triggered by
the ro->rw transition causes remove_and_add_spares() to run while mdmon
is attempting spare activation.
Result is that the kernel races to set the slot immediately after
sysfs_add_disk() writes new_dev. mdmon thinks the spare activation
failed and declines to send the monitor a new acitve_array. We show
degraded after the wait because the monitor cannot notify the metadata
that all disks are in_sync.