librbd/cache/pwl: cancel advance dispatch of external flush request

librbd/cache/pwl: cancel advance dispatch of external flush request

For external flush request, it new syncpoint after passing
guardedrequest and before dispatch. Then dispatch bypass deferred
queue But the last write request may still in the deferred queue.
It don't dispatch and not associated with any syncpoint. The
external flush request will bypass the previous write request in
deferred queue now. This does not conform to the semantics of
external flush requests. External flush request should strictly
follow the order of dispath.

But for internal flush request, it will be dispatched after all
write request which associated with previous syncpoint, persisted
in cache. C_gather guarantee it.

It is necessary to distinguish between external and internal
flush requests. Internal flush can and should be dispatched in
advance bypass deferred queue. At the same time, the order of
external requests needs to be kept unchanged. So cancel advance
dispatch of external flush request.

Fixes: https://tracker.ceph.com/issues/52599
Signed-off-by: Yin Congmin <congmin.yin@intel.com>