md-cluster: wakeup thread if activated a spare disk

When a device is re-added, it will ultimately need
to be activated and that happens in md_check_recovery,
so we need to set MD_RECOVERY_NEEDED right after
remove_and_add_spares.

A specifical issue without the change is that when
one node perform fail/remove/readd on a disk, but
slave nodes could not add the disk back to array as
expected (added as missed instead of in sync). So
give slave nodes a chance to do resync.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>