Merge PR #21374 into master

* refs/pull/21374/head:
	qa: add test for snap format upgrade
	mds: initialize SnapServer::snaprealm_v2_since after journal replay
	mds: properly distinguish cap update from snap flush
	mds: update dev document of cephfs snapshot
	doc: add release notes for cephfs snapshot
	mds: allow snapshot by default for new filesystem
	mds: close past parents after snaprealm format gets converted
	mds: automaticly allow multi-active MDS after scrubbing all inodes
	mds: don't mark primary dentry damaged if inode has been repaired
	mds: upgrade snaprealm format during scrub
	mds: allow scrubbing mdsdir
	mds: cleanup scrub code
	mds: show health warning if multimds with old format snapshots
	mds: automaticly allow multi-active MDS after removing all old snapshots
	mds: disallow multi-active MDS if snapshot was ever created by pre-mimic mds
	mds: validate SnapInfo::long_name before using it
	mds: don't bump snaptable last_snap when renaming snapshot
	mds: properly save snaptable after upgrading version

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

diff --cc PendingReleaseNotes
Simple merge

diff --cc src/include/ceph_fs.h
Simple merge

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

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

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

diff --cc src/mds/MDSMap.h
Simple merge

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

diff --cc src/mon/FSCommands.cc
Simple merge

diff --cc src/mon/MonCommands.h
Simple merge