librbd/cache/pwl: avoid stack overflow caused by nested shared_ptr destruction

librbd/cache/pwl: avoid stack overflow caused by nested shared_ptr destruction

Destruction of nested shared_ptr will cause stack overflow.
With the explicit assignment of nullptr, the deleted node
is completely disconnected from the current linked list

-------              *******               -------
|sync | <--earlier-- |sync | <--earlier-x- |sync |
|point| --later----> |point| --later----x> |point|
-------              *******               -------
   |                    |                     |
   V                    V                     V
-------              -------               -------
|log_ | ---next----> |log_ | ---next----x> |log_ |
|entry|              |entry|               |entry|
-------              -------               -------

earlier: earlier_sync_point
later:   later_sync_point
next:    next_sync_point_entry

Fixes: https://tracker.ceph.com/issues/51418
Signed-off-by: Feng Hualong <hualong.feng@intel.com>