[Sheepdog] [PATCH] Remove unregister_event from sd_xxx_handler()

[Christoph Hellwig]

I have looked over the patch a bit more and I think there is an issue
in it as it decouples the epoch update formerly in __sd_leave_done
from the vnode information update the line below - it seems like we
might now hit a window where the vnode has alreayd been incremented
by operations still operate on the old vnode confifuration.  But at
least to me it seems like we could update the vnode configuration
earlier, too.

another thing I noticed is that I think your patch makes the ->dispatch
cluster driver operation obsolete.  Once we never register the event
handler for the cluster driver FD it can be handled netirely inside the
cluster driver, e.g. just call register_event from inside the init
method and don't even tell the core about it.