Merge PR #25621 into master

* refs/pull/25621/head:
	mds: allow boot on read-only
	mds: setup readonly mode for PurgeQueue
	mds: return string_view for type str
	mds: add missing locks for PurgeQueue methods
	mds: delete on_error context on des

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

diff --cc qa/tasks/cephfs/test_damage.py
Simple merge

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