rgw: fix memory leak in RGWHTTPManager thread cleanup

rgw: fix memory leak in RGWHTTPManager thread cleanup

Fix memory leak detected by AddressSanitizer in unittest_http_manager.
The test was failing with ASan enabled due to rgw_http_req_data objects
not being properly cleaned up when the HTTP manager thread exits.

ASan reported the following leaks:

  Direct leak of 17152 byte(s) in 32 object(s) allocated from:
    #0 operator new(unsigned long)
    #1 RGWHTTPManager::add_request(RGWHTTPClient*)
       /ceph/src/rgw/rgw_http_client.cc:946:33
    #2 HTTPManager_SignalThread_Test::TestBody()
       /ceph/src/test/rgw/test_http_manager.cc:132:10

  Indirect leak of 768 byte(s) in 32 object(s) allocated from:
    #0 operator new(unsigned long)
    #1 rgw_http_req_data::rgw_http_req_data()
       /ceph/src/rgw/rgw_http_client.cc:52:22
    #2 RGWHTTPManager::add_request(RGWHTTPClient*)
       /ceph/src/rgw/rgw_http_client.cc:946:37

  SUMMARY: AddressSanitizer: 17920 byte(s) leaked in 64 allocation(s).

Root cause: The rgw_http_req_data class uses reference counting
(inherits from RefCountedObject). When a request is unregistered,
unregister_request() calls get() to increment the refcount, expecting
a corresponding put() to be called later.

In manage_pending_requests(), unregistered requests are properly
handled with both _unlink_request() and put(). However, in the thread
cleanup code (reqs_thread_entry exit path), only _unlink_request() was
called without the matching put(), causing a reference count leak.

The fix adds the missing put() call in the thread cleanup code to match
the reference counting pattern used in manage_pending_requests().

Test results:
- Before: 17,920 bytes leaked in 64 allocations
- After: 0 leaks, unittest_http_manager passes with ASan

Fixes: https://tracker.ceph.com/issues/74762
Signed-off-by: Kefu Chai <k.chai@proxmox.com>