qa/tasks/cephfs/test_sessionmap: reap connections immediately

qa/tasks/cephfs/test_sessionmap: reap connections immediately

We have to reap connections promptly for this test to work.

This test was broken indirectly by d51d80b3234e17690061f65dc7e1515f4244a5a3,
which moved the counter decrement to reap time instead of mark_down/stop
time.

The reaping is asynchronous, so allow for a delay in the count change.

Fixes: https://tracker.ceph.com/issues/50622
Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit c8c5071dcd4b0b788f5e924a678095ce5dc1d7f8)
Signed-off-by: Gerald Yang <gerald.yang.tw@gmail.com>