Merge PR #21167 into master

* refs/pull/21167/head:
	mds: make stopping mds export strays and subtree at the same time
	mds: handle client requests while mds is stopping
	mds: check projected linkage before migrating stray dentry

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

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

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