Merge PR #28363 into master

* refs/pull/28363/head:
	osd/ClassHandler: use std::variant for storing func
	osd/ClassHandler: no need to add `struct` before struct type
	osd/ClassHandler: use ceph::mutex instead of Mutex
	osd/ClassHandler: remove unused function
	osd/ClassHandler: s/string/std::string/ in header

Reviewed-by: Samuel Just <sjust@redhat.com>
Reviewed-by: Radoslaw Zarzynski <rzarzyns@redhat.com>