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

Fixes: #9562

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 8dc94a2d8ce3364c0d8d52f634e5fc967e43d819)
Merge pull request #2612 from ceph/wip-9562-backport

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