Fix memory leaks in librbd persistent write log (PWL) cache discard operations by properly completing and deleting request objects.

ASan reported the following leaks in unittest_librbd:

  Direct leak of 240 byte(s) in 1 object(s) allocated from:
    #0 operator new(unsigned long)
    #1 librbd::cache::pwl::AbstractWriteLog<librbd::MockImageCtx>::discard(...)
       /ceph/src/librbd/cache/pwl/AbstractWriteLog.cc:935:5
    #2 TestMockCacheReplicatedWriteLog_discard_Test::TestBody()
       /ceph/src/test/librbd/cache/pwl/test_mock_ReplicatedWriteLog.cc:534:7

  Direct leak of 240 byte(s) in 1 object(s) allocated from:
    #0 operator new(unsigned long)
    #1 librbd::cache::pwl::AbstractWriteLog<librbd::MockImageCtx>::discard(...)
       /ceph/src/librbd/cache/pwl/AbstractWriteLog.cc:935:5
    #2 TestMockCacheSSDWriteLog_discard_Test::TestBody()
       /ceph/src/test/librbd/cache/pwl/test_mock_SSDWriteLog.cc:533:7

Plus multiple indirect leaks totaling 2,076 bytes:
- DiscardLogOperation (224 bytes x 2)
- SyncPoint (184 bytes)
- SyncPointLogEntry (168 bytes)
- DiscardLogEntry (152 bytes x 2)
- Various string and vector allocations

SUMMARY: AddressSanitizer: 2316 byte(s) leaked in 15 allocation(s).

Root cause analysis:

1. Missing request completion: C_DiscardRequest objects were never deleted because their complete() method was never called. The on_write_persist callback in setup_log_operations() called complete_user_request() to invoke the user's callback, but didn't call complete() on the request itself to trigger self-deletion via Context::complete().

   Write requests use WriteLogOperationSet which takes the request as its on_finish callback, ensuring complete() is eventually called. Discard requests don't use WriteLogOperationSet and must explicitly call complete() in their on_write_persist callback.

2. Sync point not reset: m_current_sync_point was never explicitly reset during shutdown, keeping the sync point alive even after m_log_entries was cleared. This prevented proper cleanup of the sync point chain.

3. Indirect leaks: C_DiscardRequest holds shared_ptr, which holds shared_ptr, which holds shared_ptr to log entries. When the request leaked, all referenced objects also leaked through the shared_ptr reference chain.

Solution:

1. Add discard_req->complete(r) in the on_write_persist callback (Request.cc:445) to properly delete the request object after completing the user callback and releasing the cell. This follows the same pattern as WriteLogOperationSet for write requests.

2. Add m_current_sync_point.reset() in shut_down() (AbstractWriteLog.cc:682) after m_log_entries.clear() to explicitly release the sync point and break the reference chain.

These fixes ensure proper cleanup of all allocated objects in both production and test environments.

Test results:

• Before: 2,316 bytes leaked in 15 allocations
• After: 0 bytes leaked
• unittest_librbd discard tests pass successfully with ASan

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