drbd: only wakeup if something changed in update_peer_seq

This commit got it wrong:
    drbd: Make the peer_seq updating code more obvious

    Make it more clear that update_peer_seq() is supposed to wake up the
    seq_wait queue whenever the sequence number changes.

We don't need to wake up everytime we receive a sequence number
that is _different_ from our currently stored "newest" sequence number,
but only if we receive a sequence number _newer_ than what we already
have, when we actually change mdev->peer_seq.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>