mdmon: fix, close spare activation race

mdmon: fix, close spare activation race

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.

#!/bin/bash
i=0
false
while [ $? == 1 ]
do
	i=$((i+1))
	mdadm -Ss
	mdadm -CR /dev/md0 /dev/loop[0-2] -n 3 -e imsm
	mdadm -CR /dev/md1 /dev/loop[01] missing -n 3 -l 5
	mdadm --wait /dev/md1
	mdadm -E /dev/loop2 | grep -i degraded
done
echo "failed: $i"

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>