bcache: avoid deadlock between makeRoomForSize and onEvict
bcache: avoid deadlock between makeRoomForSize and onEvict

The deadlock:

* golang-lru takes its internal lock, calls b.onEvict, which takes bytesLock
* b.makeRootForSize takes bytesLock, calls cleanTransient.Len(), which takes
  the golang-lru internal lock.

Instead, only access cleanTransient.Len() outside of the bytesLock
critical section.

Issue: KBFS-1336

keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client