md: Keep /proc/mdstat reporting recovery until fully DONE.

Currently when a recovery completes, mdstat shows that it has finished
before the new device is marked as a full member.  Because of this it
can appear to a script that the recovery finished but the array isn't
in sync.

So while MD_RECOVERY_DONE is still set, keep mdstat reporting "recovery".
Once md_reap_sync_thread() completes, the spare will be active and then
MD_RECOVERY_DONE will be cleared.

To ensure this is race-free, set MD_RECOVERY_DONE before clearning
curr_resync.

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