logd: update region lock after entry has passed to reader socket

(cherry pick from commit de4bb9c1)

- The reader region level indicates the location to protect, but once
  it has been passed to the calling reader, then allow us to go
  one beyond so that a clear or prune thread can remove the entry.

Bug: 23711431
Change-Id: I0f2389858dd8c83366c034360f67d7c363625b56