mds: give up replicas of a stopping mds's stuff

mds: give up replicas of a stopping mds's stuff

In order for an MDS to make it through stopping when
it had some strays, the other ranks that serviced
the migrate_stray renames must ensure that they
give up any cache objects that belonged to
the stopping MDS, so that the stopping MDS
can finish emptying its cache.

Fixes: #10744
Signed-off-by: John Spray <john.spray@redhat.com>