Merge PR #17452 into master

* refs/remotes/upstream/pull/17452/head:
	Make standby-replay mds not migrate empty subtree when trimming cache
	Make standby-replay mds avoid initiating subtree export

Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>
Reviewed-by: Zheng Yan <zyan@redhat.com>

diff --cc src/mds/MDCache.cc
Simple merge

diff --cc src/mds/Migrator.cc
Simple merge