mon/OSDMonitor: trim no-longer-exist failure reporters

mon/OSDMonitor: trim no-longer-exist failure reporters

remove the report from failure_info in OSDMonitor::check_failure(), if
the reporter does not exist in the osdmap any longer. otherwise, we will
run into assert() failure when trying to lookup the osd in osdmap using
`osdmap.get_xinfo()`.

Fixes: https://tracker.ceph.com/issues/41680
Signed-off-by: NancySu05 <su_nan@inspur.com>
(cherry picked from commit 99ccc166ede5f296b86abfe9db0a3d597100edde)

Conflicts:
	src/mon/OSDMonitor.cc
- ceph_assert, g_conf()
- account for C++17ism by moving iter initialization out of if statement