snaps on kclient
+- snap-ordered writeout
+ - switch i_wrbuffer_ref to int, protected by i_lock
+ - split out i_wrbuffer_ref_head
+ - on queue_cap_snap, _head count to new snap and reset
+ - in writepages, take i_lock and
+ - lock for oldest cap_snap with flushing > 0,
+ - and make sure our page is for that snapc. once we have a page, the snapc has to match.
/- we need a rw snap mutex, so that pending io blocks while we update snap contexts
- we can push down to a per-realm rw mutex later
/- need to do ORDERSNAP flag