Fixed locking issue in the trun method of libradosstriper leading to potential race conditions - Fixes: #10129

Signed-off-by: Sebastien Ponce <sebastien.ponce@cern.ch>
Merge pull request #2951 from sponce/master

Fixed trivial locking issue in the trunc method of libradosstriper

Reviewed-by: Loic Dachary <ldachary@redhat.com>