monitor: don't unblock a device that isn't blocked.

monitor: don't unblock a device that isn't blocked.

When we see a failed device, we both unblock and remove it (after
updating the metadata).
However it might not be blocked as there can be a delay between
unblocking and the device being free to be removed.

If this happens the clearing of 'blocked' succeeds so md sends a sysfs
notification and mdmon checks again and tries to clear 'blocked'
again.
Thus it enters a busy-loop until the 'remove' succeeds.

To avoid this, only try to unblock if the device was blocked.

Signed-off-by: NeilBrown <neilb@suse.de>