librbd/cache/pwl: don't clear next_sync_point_entry prematurely

librbd/cache/pwl: don't clear next_sync_point_entry prematurely

In SyncPointLogOperation::clear_earlier_sync_point(),
sync_point->log_entry->next_sync_point_entry was prematurely set to
nullptr in clear_earlier_sync_point(). It is in write op stage, but
next_sync_point_entry is used in writeback stage in
handle_flushed_sync_point().

handle_flushed_sync_point() may pass a nullptr
cause assert in m_work_queue.The solution is to move the statement
that set next_sync_point_entry to nullptr after it is used.

Fixes: https://tracker.ceph.com/issues/52465
Signed-off-by: Yin Congmin <congmin.yin@intel.com>