client: flush the caps release in filesystem sync

client: flush the caps release in filesystem sync

We have hit a race between cap releases and cap revoke request
that will cause the check_caps() to miss sending a cap revoke ack
to MDS. And the client will depend on the cap release to release
that revoking caps, which could be delayed for some unknown reasons.

In Kclient we have figured out the RCA about race and we need
a way to explictly trigger this manually could help to get rid
of the caps revoke stuck issue.

Fixes: https://tracker.ceph.com/issues/67221
Signed-off-by: Xiubo Li <xiubli@redhat.com>