The new_disk_ack could return failure if WAITING_FOR_NEWDISK
is not set, so we need to kick the dev from array in case
failure happened.

And we missed to check err before call new_disk_ack othwise
we could kick a rdev which isn't in array, thanks for the
reminder from Shaohua.
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>