osdc/Filer: drop probe/purge locks before calling objecter

Fixes: #9562

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2572 from ceph/wip-9562

osdc/Filer: drop probe/purge locks before calling objecter

Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Yan, Zheng <ukernel@gmail.com>